I would go 17x8 all around 4-1/2" backset.
I have 245 45 17 all around. With stock suspension. Car is lowered about 1"-1-1/2" in front. Front is close but fits, plenty of room in back. If I was to do again I would get 9" wide in back and go with a wider tire also.
I was on there website and they only have small block crate engines available.
I may need to call them and make sure they are still building big block engines.
The only issue to extending the seat back farther with an extender, is you might hit the seat belt retractor that's bolted to the floor. 70 model. Push your seat all the way back and check the clearance from back of seat to seat belt retractor.
Someone posted this on another site. Looks interesting
SST new 5 speed for e-bodies
Silver Sport Transmissions Unveils The Mopar E-body Tremec TKX - Mopar Connection Magazine | A comprehensive daily resource for Mopar enthusiast news, features and the latest Mopar tech