1994/1995 transmission swap

While I like the idea, unfortunately I can tell you the swap will involve far more work than you would think. GM changed pretty much everything in the driveline sensing and electronics between those 2 years. The switch being moved is only part of it, the entire engine control system is different as well because the 1994 is an OBDI vehicle while the 95 is one of the early OBDII systems and even then it isn't fully compliant. As such you would need to change not just the engine and transmission but most of the electronics to make them work with the 94. The PCM and it's harness as well as the transmission harness and even the dash is different because of the changes in those 2 years. It makes it a difficult thing to work on the 95's simply because of the halfhearted attempt GM used on a few models that year, plus because of it, many 1995 parts fit only the 95 and don't work with other years.

Yeah GM really messed with things in those years. Up to 94 things are simple, after 96 they are not that bad, but 94-95 can make you learn words that even soap won't wash out. What GM did was different than other companies, Ford for instance knew just like all of them that OBDII was going to be required in 96, so when they started on a design to be released in late 95 or 95 they built it with the full OBD II standard in place so it was already set to go.
GM on the other hand put together a few models that were intended to be new for 95 - up but instead of going to fully OBD II standard, they added some of the OBD II pieces to the existing OBD I system, then added additional electronics and hardware that makes them unique. Most mechanics who work on them call it OBD 1.5 and some other names I won't post! The problem is that you have to use a scan tool to work on them, and that scan tool has to run specific software to even work. They used two different ALDL connectors which only match the 95 model year first, then the software itself will not work with an OBD I scan tool, and most OBD II tools cannot read it either. Plus just to make it even more interesting, they didn't do this with every option package for a given vehicle. So you could get a 95 Chevy truck with say a 5.7 and automatic that was still OBD1 but a 4.3 version of the truck was OBD 1.5. Same in the cars, I have a couple cheat sheets from that time showing which ones do what but if at all possible I try to stay away from them because of the issues.

Personally if I had a nice earlier truck and wanted a modern engine I would probably do an LS swap with it's transmission, these days all of the mounts and such are available as kits along with a stand alone PCM that gets set up to feed the factory dash panel. It's a bit more work but once done you get a much improved vehicle. The other option is to build the 94 engine into something better, which can be done for a price.

Engine is a maybe, 95 was the year they added the roller cam in some engines and it was a different grind from 94 so that can cause issues. The only way to tell for sure is to look at the casting numbers on the block and heads and cross those. The 4.3 is a 95 only version, the 5.0 casting number 627 and 2187 heads will work okay if it isn't a roller cam, the 5.7 though is all over the place as it was offered in both versions with a roller and without and on those it depends on which option package it had.

Okay, those would be the 4.3. Basically a 350 with 2 cylinders removed. Actually a good engine. However the 95 is 1995 only as is the transmission, unless everything else gets changed with it, IE all the electrics, that is because of the changes GM made that year. The 94 block is different as well because the 95 uses a balance shaft along with a different cam. You might be able to make it work after a fashion using just the engine, but I'm not sure if the sensors are actually all the same. The transmission isn't going to work though as the internal electronics are different enough to cause the PCM to go nuts.