transmission swap

I have a th 350 transmission to swap into my 88 caprice classic and was wandering if there will be any modifications i will have to make to get it to work

The crossmember - you will need a crossmember for a TH700-R4 Caprice to make it work. The driveshaft should be good for the TH350 if your car has a TH200-4R in it - as long as your TH350 has the 6" shaft, both are the same length. You will also need to route a vacuum line from the intake (needs to be intake vacuum, not ported vacuum) to the vacuum modulator on the transmission (which the later transmissions don't have). You'll also need the detent cable. It connects to the carburetor using the same brackets the existing TV cable does. The TH200-4R is actually about an inch longer than a 6" TH350, but you should have enough driveshaft yoke that you shouldn't have to worry about it.

After all of this, my question would be Why?

Upgrade it to a TH700-R4. The 700 is based on the TH350 and it has overdrive, a salient point in today's world of $2.50/gal gas prices. As long as you get the 1987 or newer version of the TH700-R4 (beginning in 1987, the OFFICIAL designation for this transmission from GM is 4L60) you should be good to go. Some people consider the following to be a negative point with the TH700-R4/4L60(e) - the short first gear. If you're racing it, then it is a negative point, but the 700-R4 has a 3.06 first gear vs. the TH350's 2.52 first gear. With your 305, this will translate into better acceleration through first gear (read: takeoffs) than with any other available automatic.

To clarify the negative point regarding the 1st gear isn't so much about the 1st gear ratio, but much more about the spacing of first and second gears - 2nd is much taller and in racing, this causes the engine to drop below its powerband - basically causing a race loss by the car "falling on its face".

To answer you question about why.. its the only transmission I have laying arround, and its not going to be a daily driver. Right now the car is waiting to have a freshly rebuilt 396 dropped in. And I would rather not use a 700R4 because the motor should have about 420 lbs/ft at about 2500 rpm and I'm told that the 700 wont handle that kind of torque unless I put a lot of money into it.

Well you're more than likely going to run into the same sort of trouble with the TH350 - you'd want a TH400 or a built TH700-R4 to handle the torque of that beast. The TH200-4R can also be made to handle that power.