Installing a "corporate" TPI engine-which was originally installed in third-gen Camaros and Firebirds and C-4 Corvettes-in your GTO would be fairly simple and a lot less painful than dealing with the Pontiac faithful the first time you open your hood at the local cruise. TPI engines are pretty much the same as other Chevy small-blocks, with the main difference being the use of the one-piece real-main seal beginning with the '87, which only requires that a compatible flexplate be used. To drop it in, you could simply find a set of Chevelle small-block frame mounts and bolt them to your GTO's frame, which most likely already has the proper holes (most GM intermediate frames of the era were drilled with all the possible engine-mounting holes). Then, bolt a set of the proper engine mounts to the TPI small-block and swing it in. If you use the TH700-R4 automatic trans that accompanied the TPI engine, you'll find its tailshaft is even longer than that of the TH400 that the GTO probably has now. You may be able to accommodate this by sliding the transmission crossmember farther back on the frame and making new mounting holes. However, if your car has the BOP-style crossmember with the rubber isolators at the ends, you should probably find a Chevelle-style crossmember, which is tubular and bolts directly to the frame. In some cases, sliding the crossmember back on the frame won't be an option, either because of obstructions, or because it will raise the tail of the trans too high. In these cases, modifying the crossmember may be a better approach. Most swappers going this route will cut the mounting pad for the trans mount off the front of the crossmember and weld it to the back, which, when coupled with minor frame positioning adjustments, will usually do the trick. It's important, however, to maintain the proper operating angles on the driveshaft, which will have to be shortened and fitted with the proper slip-yoke for the TH700-R4. The simplest way to ensure this is to make note of the angle of the original transmission's tailshaft and maintain the same inclination with the new trans.
Once the engine is in, you'll have to tackle the task of adapting the engine's electronics to your car. For this, you can adapt and modify a stock Camaro/Firebird engine harness, or you can use a swap harness, as offered from sources like Street & Performance (which also offers videos on TPI swaps) or Painless Performance.
If you decide to stay with your existing engine, you can still swap in an overdrive automatic. Unfortunately, if you're running a traditional Pontiac V-8, the TH700-R4 will not bolt directly up, as it is only offered with a Chevy-style bellhousing bolt pattern. There are adapters available (Advanced Adapters), or you could use a GM TH200-4R overdrive trans. The TH200-4R was used in many Olds V-8-powered models during the '80s, though it was also used behind the Chevy V-8 in certain applications. For this reason, most 200s have a multi-case arrangement, with both Chevy and B-O-P bellhousing bolt patterns. The TH200-4R also benefited from its use behind Turbo Buick V-6 engines, as the factory strengthened the design to handle the extra output. The aftermarket took over from there, so it is possible for the TH200-4R to handle the power of your Poncho. Dimensionally, the TH200-4R is the same length as a short-tail TH350 or a Powerglide, and the rear mount is in the same location as a TH400. It also uses the same driveshaft slip-yoke as a TH350 or Powerglide, so the physical part of the swap shouldn't be difficult.