Another tip, if you do run into troubles with the length of the motor, you can try to get a 6 cyl radiator and core support. This will give you maybe 4" of extra clearance in front of your motor. Also, the 6 cyl radiator will work fine with a 302 or 351. Another benefit to using the 6 cyl rad is that is has a single inlet and outlet, unlike the flatty V8's rad that has 2 and 2.

I have a big block Ford (429-460 style) in my 50 and it fits very well.