The engine set up is relatively simple, you may have to add an electric fan to the radiator (thinking about doing it to my truck, 73 Ford with a 91 5.0 EFI), as it will take a lot longer to warm up. The major problem lies with the motor mounts. If your engines are original, you either have the 389 or the 421 and I don't know how well they'll translate to another motor mount. Usually, the easiest way of going about it is getting a donor car with the engine that you're looking for and pull the motor, tranny, and ECU, if you're going with OBD I, the transmission is more for the convenience of not having to get the correct match since OBD I doesn't look at the transmission where with OBD II, the transmission does give feedback and things become a little more complex. There are kits, however to get around this, but, once again, things are becoming a little more complex, for the first time around, I'd suggest keeping it simple. While the engine's out, I'd take the time to do a little build\rebuild with it, but that's up to you. Going this route, the only things you have to really be concerned with for the adaptation is the motor mounts and probably the drive shaft, and custom drive shafts are relatively cheap unless you're pushing 600+ HP.
Going to the floor with the shift and bucket seats will probably be 100% modification unless the Cat shared a floor pan with something that shifted from the floor, then an access cover may be on the hump, otherwise, you're cutting a hole. What I'd recommend is going with a cable shift setup, so the only restriction for where the shifter goes is the seat and console. Given the size of the car, you might want to look into a truck with a center console and bucket seats and see how close the seat frame matches up, modifying the mounting of the seats are relatively simple. BTW, my truck is 5 on the floor with bench seats, doesn't create a problem unless someone is sitting in the middle, then we're really friendly when going into 2nd, 4rth, and reverse.
As far as simple, hard, or impossible, well, impossible is not in this picture. Simple and hard depends on your capability and hopefully with what you need to look at can help you determine the answer.