There are several possible sources of
oil leaks at rear of 351w
1. rear valley corks of intake manifold
2.
oil gallery plugs at rear of block (3)
3. Large core plug at rear of camshaft bore
4. Sump seal around rear main cap
5. Rear main cap if sealant has not been applied between cap & block
6. Rear main seal- if two piece type- three likely causes, 1.ends of seals should be staggered in relation to cap parting line. 2. pin or hole for use with 'dogs turd' type rear seals, if this pin is not removed or the hole for it not filled & deburred it will distort the lower seal half at this point. 3. check that drain hole in rear main cap is open, I have seen some come from factory where they have not been opened up enough by the machine work which floods the rear seal.
7. Thread sealant reqd on Flywheel bolts- this is not reqd on early cranks as holes are blind, but later single piece seal cranks may be open and therefore require sealant.
8. Grease or lube should be applied at the crank/seal interface during assy, if its simply oiled and then put in storage for some time prior to startup its not uncommon for the seal to 'stick' to the crank and destroy the seal surface and it will have a slight leak from thereon.
Two piece remove the pan & rear main cap to replace---single piece, remove trans, clutch & flywheel to replace.