A 385 series big block (429/460) will fit. As far as displacement, your wallet is the only limiting factor. A good production 460 block with sufficient cylinder wall thickness can be built to 557 cubic inches without even having to do any clearancing. Aftermarket blocks with a standard 10.3" deck height can be built to well over 600 cubic inches.
a 460 will go in there.. the 69 model was the first satng to be de
signed to receive a bb without alot of trouble.. the 73 is better yet..good luck.. remember your suspension and dirvetrain will need help as well.