The exhaust set up for a big block duster is somewhat limited.
If you go with a B instead of RB, (383,400 vs 413-440) you will have more exhaust clearance to work with which on these cars is critical as there is limited exhaust room as it is.
All exhaust choices except the stock manifolds involve major$$$$$
Fendrewells: have seen may fenderwell sets that required the use of skinny front rims and tires to provide enough tire clearance so that they won't rub
Traditional headers with "equal length tubes:very tight fitting with various clearance problems
Non traditional such as Schumacher: probably the best fitting out of the box, however they are the "tri y" design without all tubes being equal length and even though they will bolt on and fit fairly well, their design is not conducive to high horsepower high winding application but better suited for low and mid range operations
B and C body big block manifolds: I believe that milling the B driver side 1/16 of an inch will allow you to get a set in an A body without major frigging
On a budget---stock manifolds
Low frigging and chiseling factor---Schumaker