I know it is not a TA or Formula but the car has been with us since 1980. The current 350 is shotand the th350 needs a rebuild. I have a '73 455 with a th400. I have torn down the 455 and the cylinders look good. It has been over bored before to .030. The th400 is a total unknown both were pulled before I bought them. First question should I go ahead and use the 455 to replace the 350 or rebuild the 350? Second, with tranny stick with the th350 or swap to the th400? I now the drive shaft would need shortening but what else is involved? I'm not planning on dragging this car everyday just getting her back on the road with more power and maybe the ocasional track trip or stop light to stop light rice burner blow away.  

Whew, had to re-read the title of your post, I thought you said "Sunbird"! That would be a shoe-horn deal! Assuming your Skybird has a pontiac 350 not a chevy, the 455 should be an easy bolt-in deal including using the th350. Whether you spend the money rebuilding the th350 or th400 is up to you. If you use the 400 trans, the driveshaft needs to be shortened and needs a larger front yoke for the 400's larger output shaft. The crossmember works if you redrill the holes in the subframe (they may be there already, mine were) and you need the switch and wiring for the electric kickdown as well as the correct shifter cable bracket for the th400. If you truly are going to drive it reasonably, the th350, properly rebuilt should hold up fine assuming you stick with a mild engine build and don't try running slicks at the track every weekend!
Your Skybird is a unique car in a sea of TA's and Formulas, if it is really nice and you want to be able to put it back to original someday, go with the 455 and put the 350 away after oiling the cylinders. You likely have some serious highway gears in the rear, so the 455 will be amazingly driveable and have lots more power than the 350 had.
Steve