I tried to copy a page out of a '64 Pontiac manual about packing rear seals, I couldn't get to work so here's what Pontiac did. This is with crank out of the block, the crank will need to be away from the block to install the seal with out stretching it, it needs to go in loose. Once you have it in the block, tighten the main caps to push the seal into place.
Pontiac used this method for all V8's from the start to the last one in 1979. 3" and 3.25" crank journals, seals were different size but installed the same way.
Make a tool out of 1/2" brass rod with the packing end matching the groove in the block.
The packing portion should be 3/4" long.
Install the seal without stretching it and trim even with block after pushing it into the groove with tool or crankshaft.
With the crankshaft firmly in place, pack the seal down with the tool you made till it bottoms outs ( 3/4" ) per side.
Carefully install the seal in the cap not stretching it and use a socket or round tool to compress it into the groove. A slight compression of the seal is recommended.
Trim off excess leaving a small portion proud of the cap mounting surface.
Form the ends to small mounds or pyramids.
Cut the excess seal material into four 3/8" long pieces.
Pack each side of the seal in the block with one 3/8" piece with the tool going half the distance as the first packing.
Pack the second 3/8" piece on each side leaving a small portion proud of the block deck.
Form the ends into mounds or pyramids so the seal stays off the mounting deck when assembled.
Apply a small portion of sealer to the deck around the seal area but not on the seal.
Assemble the rear cap and torque to spec's.
The ideal behind the packing is as the engine runs, the seal will start to unpack forcing its way around the cap compressing it to make a tighter seal. Only the old style seals will take to packing, the new stuff will just fall apart. Best gasket material does not take to packing, its not needed. I find if I compress the seal as I put it into the grooves, it works better than just laying it in.
