Sorry, bb1979. I overlooked your question for piston sources. New pistons and internal parts are almost non-existant. There are places that will custom-build them. All parts new, or used are pretty pricey. When I was looking for pistons, a salvage yard told me he sold used pistons for $80. each, but didn't have any. I was never able to find any source with anything in stock. Good source for info and leads is
www.gmcguy.org.
Exhaust manifold: This is what I finally came up with to get past the steering box. I'll probably need to wrap it with insulation as it still runs close to valve cover and steering box. I cut the stacks, connected them, then down & out. Steering box sits in front of pipe. Crude and rough, but hope it works. For now, will use stock manifolds on the other three.
Tranny: Another guy is working on an adapter for an T400 or 4l80E auto. It looks good so far, but not finished. An auto would sure simplify things.