It is now time to get the transmission ready. If you are installing a TH 350 trans,
look at the top of the bellhousing surface where it will touch the top
of the engine block right where the topmost Stude bellhousing bolt would be. 
On some trans cases you will need to remove part of the "web" that was
cast into the Chevy bellhousing. Remove enough to clear a bolt head without
removing the whole web. Sometimes you can simply break off a small
piece with a crescent wrench or pliers, other times if the web is more
substantial (some may be solid), you may have to file a notch.
Do NOT skip this step. The transmission MUST sit flat
against the engine.
You can do all the filing inside the bellhousing where it will not
 be seen from the ouside.

Looking at the transmission case, locate the two large 5/8 inch
GM dowel pin holes.
The two lowest bellhousing bolts, just below the dowel pins, will have to be
filed with a coarse rat-tail file upward and slightly inward to
allow the lower bolts to thread straight into the Stude block.
Another way would be to drill the housing holes oversize to about 9/16
I prefer the filing method so you don't remove so much metal and
then have to use large washers under the bolt heads.
Filed holes will make a cleaner installation.
Using washers under all the bolt heads where the bolts
touch the aluminum housing is a good idea.
 
Selecting a torque converter---- 

  Generally speaking,  the converter stall speed is related to the converter
size and the engine torque. 
The stall speed is NOT, repeat it is NOT the rpm's
your engine will be running while driving!
A 10 inch 3,000 rpm stall converter will NOT make your
engine spin 3,000 rpm while driving.

It will still act and feel like any stock converter under normal
driving conditions.
A 3,000 stall converter gets it's name from the fact that with a healthy
Chevy 350 smallblock, in gear, with the brakes locked, AT FULL THROTTLE
 until the car feels like it is internally hemorraging, will
probably be able to force the converter to slip to 3,000 rpm for a time.

Put that same converter on a 283 or 289 and it will suddenly turn into
an aprox 2,000 rpm stall speed converter (very close to many factory
stall speeds). 

A converter with too low a stall speed will make your car feel like it needs
extra brake pedal pressure to keep it from inching forward at stop lights,
and will likely have a rough in-gear idle that you can't seem to adjust out.
If you have a car that will idle nicely in neutral but runs rough while
idling in gear, you likely have a tight converter that is pulling down your
idle speed too low for the engine to run smooth.

The good news is that if you are going to use the 700 R4 4-speed,
they were installed in cars with smog junk and smaller engines.
 The smog cars usually had a lot less low end torque than the older
 Chevies without the smog equipment. That means most 700 R4
converters are already "loose enough" to use on the smaller Stude engines.

If you will be using an older TH 350 or TH 400 just remember that
most of those were used behind bigger V8s and had the higher low-end
torque of the pre-smog engines.
Many came with large 12 or 13 inch converters that would feel tight even
on a 455 or 400.
Many years ago, one of the very first TH 350 swaps I did was on a Stude 232.
I left the stock 12 inch converter in place (probably from a Chevy 350 pickup).
The little 232 idled great until I put it in gear. The idle was pulled way down to
where the engine would run rough. Everything operated fine except for the
really lousy idle.  Later when I changed to one of the smaller (10"?)
6 cylinder converters, it acted so much better.
Don't worry, the 6 cyl converters are just as tough as the V8

 My whole point is that you should check to make sure that  your
TH 350 or TH400 doesn't come with a converter that is too big
for a Stude sized engine.

12 or 13 inch is too tight unless you have very good low-end torque.
A 12" would be ok for a good 289 family car.
11" for a 259 in a normally operated family car.

11" stocker or one of those $110 "Saturday Night" specials
(slightly loosened stock conv) is probably right for a mild-hot-rod with a
stock cam. 
10" is about right for the smaller Stude engines or a 289 with a "cam".

Don't spend a lot of time worrying and fretting about getting the exactly-
correct one. There is so much overlap in the choices that if you do pick the
"wrong one", you will probably not even notice it.
You will probably only notice it if you are WAY off.
Before going to the next phase of installing the trans, make sure
the torque converter has been checked for fit with the flywheel.

Slide the torque converter of your choice on to the transmission
input shaft. It may take a bit of wiggling and rotating to get it to
slide all the way on. There are at least a couple sets of splines.
Make sure the converter is all the way on!!

Raise the transmission to the height and angle to match the engine
height and angle. As you mate the transmission to the engine,
keep a close eye on the gap between the engine and the block.
If the space is even all the way around, you will know the trans is
going in evenly and straight.

As you wiggle and push the trans into place on the studs, you should
periodically turn and wiggle the torque converter to make sure the pilot
enters the crank hub properly. Failure to do this could result in damage
to the converter or the front oil pump when the bolts are tightened.
Please resist the temptation to pull the trans into place by tightening the
bellhousing bolts. You can find it helpful to install the bolts, and as you
wiggle the trans in place, slowly turn them in to bring the trans closer to
 the engine, but do not rely on the bolts to pull them together.
Wiggle and push all you want to get the trans in to place, but do not
use the bolts to pull them together. 

You can break internal parts if you tighten the bellhousing bolts before
all the splines, and pump drive tangs line up and slide together.

Bolt the transmission to the engine, and bolt the torque converter
to the flywheel.

DOUBLE CHECK THAT THE
 TRANSMISSION IS FLAT AGAINST THE BLOCK!
Any gaps or spaces indicate some interference somewhere.
Check bolt-head clearances etc if that happens.

Do not install the dust cover yet.