the boss 302 block[69-70] had 4 bolt mains andwas stronger throughout. ford has recentlyreleased a new block called the boss 302 it is even sronger but pricey.
as far as stock block 351, i believe the later ones may have been set up for roller lifters ,other than that i would recommend a van eng. i believe they have the right oil pan double sump.
if you want more info ck a mustang sight they have tons of info