I have an 82 that I am doin the same thing to. Will be putting it in a 79 Blazer with a 4 speed. I figured I would use a hummer manifold and cut off the flanges and use gaskets on both sides of it for my spacer. I also had another idea about the downpipe situation. How about running the downpipe first and snaking the crossover in after? I realize that this would be a lot less hassle on a 4 wheel drive than a 2 wheel drive. What do you think?