First off good luck with the swap.
Next thing I want to adress is that are you sure its a 383? Imperials came standard with 440's, not 383's. Next thing is that your K-frame isn't for a big block but you have the benifit of having a spool small block k-frame which you can convert into a big block one.
Also there is no difference between 6 pak and regular heads- their the same. The only 6 pak heads that are different are the T/A AAR 340 six pak blocks.
The exhaust is the hardest part to do for the swap. You can find factory A-body big block manifolds ($$$) or I think some C-body ones fit. Then theres the header option. Shumacker makes very nice headers for big block A-bodies.
Now in my opinion I would get a 340 to put back in because you car because its an original 340 Swinger. Just my $.02