make your own motor mounts, move motor back about 12" to help distribute the excess weight, make your own exhaust. rebuild tunnel & firewall, figure out how to get the short rear drive line to work in a stock wheelbase, or move that 60 back to help you out. figure out some way to keep the motor cool, might want to go ahead and ditch the core support, since it wont flow enough air to the oversized radiator you'll need.
i have seen a few broncos in my day with big blocks in them. they are fast, sure but ill handling and nose heavy, & run hot. fun yea.
it's going to be allot of work, better get started now.
good luck and keep us posted on how it's progessing
be good
nick