I looked at putting a 251 in my 37 & heres the deal - the 251/265 are 2 inches longer.
37 bell-housing will bolt to the larger motor so no need to alter the rear mounts - you cann't shift the motor back unless you want to cut your fire wall.So you have to shift the front mount chasis brackets forward on the chasis. You have to shift the radiater forward 2 inches which is where it gets tricky. The sway bar which sits on top of the chasis gets in the way. You cann't shift the sway bar to the bottom of the chasis as there is not enough room between the chasis & the springs. I think you can shift the radiater forward as far as you can & lean the top forward to clear the fan. The reason I say fan is I have my reservations about elect. fans on these motors. Convert your 4 blade fan to a 55-59 6 blade fan this improves cooling. It's not a difficult conversion, but not a weekend swap either.
I didn't make the swap in the end not because of difficulty but a 230 became available & I picked up an alloy head & 2 carb manifold for next to nothing. In previous posts you'll see the pros & cons of 230 vs 251/265. Either way hope this helps & good luck.
