Since all the smallblock engines have the same dimensions, why would you choose a 265? Those engines were pretty puny when they were new, and 50 years of use certainly hasn't improved them. The engine mount bolt holes and bellhousing pattern are shared by all the small blocks, so why not use a 305, or even a 350? If you can locate an early V-8 bellhousing with the side mounts, and a pickup-type front horseshoe mount, the adapting will be fairly simple. The 51 never had a V-8 option, so there will be some clearance issues regardless of which engine you use. It doesn't make sense to use an antique engine when newer, more powerful ones are available.
Jerry
