I'm sure you meant 26 MPG!
I don't see why the stock 6.1 shorty headers wouldn't work in a A-body?? They dump the same way stock smallblock mannies do!
There are a few different options with exhaust for the swap. I believe the easiest fit variety are the truck manifolds (maybe Jeep, I can't remember), but I believe they dump pretty close to the firewall. They offer the best clearance near the block though. Some have had luck with the LX car manifolds, but you have to take a bit of a chunk (not real bad) out of the passenger frame rail for clearance. The shorty 6.1 manifolds have come up several times, but I think the main issue with them is the flanges. They're pretty dang big and I think they tend to get into stock steering boxes pretty quick. If you go with an aftermarket K member with rack and pinion your options open up quite a bit though.