DanM

How do I tell whether my block uses a 1 or 2 piece rear main seal? It's a 351W block, casting D6AE. Not entirely sure what year vehicle it came from (I was told 86 Crown Vic, but Im not sure) I have it all dismantled ... can I tell by looking at the block or main cap?

buzz

If it's an 86 it'll have a 2 piece main seal (late), but it doesn't matter as they're interchangeable, I'm pretty sure you'll only be able to find a gasket set with a 2 piece main seal, as the 1 piece has become obsolete.
You may be able to get (a) a rope type seal - 1 piece
(b) a rubber type seal - 2 piece
The mains cap that holds the seal has a spike in it that sticks into the rope type seal to stop it spinning with the crank , if using a rubber seal (which is probably the only one u can get) youv'e gotta take this spike out so it doesn't puncture the rubber. You just tap it out with a punch, grind the spiky bit off, and knock the rest (dowel) back into the cap to fill the hole. That's what I did to my 351W years ago with no probs.

Racer

Its actually the opposite. The one piece seal replaced the obsolete two piece.

They are not interchangeable.

blue66coupe

D6 is around 1976. If the block and cap has a groove in both NEAR the outer edge of the block then it's 2 piece if it has a large machined step to suit a rubberised 'push in when assembled' hydroelastic type seal then its 1 piece, also crank will have slash cut grooves in it for 2 piece seal and smooth finish for 1 piece