Hi Dan,
You don't need to lift your TA up on a lift to get the engine ID no.- it's behind the passenger side head on a pad, over the oil filter bracket. It's a 6-digit number, cast in raised numbers (as opposed to being engraved). It may be concealed by dirt or grease, but some cleaner and a flashlight should reveal it. I've heard the same thing too about early 78 W72's- hopefully you got a 481988 block. If it's t'he 500557 block you're still good to 450 hp, but stroking is out. The 500557 400's are just too weak in the main saddle are to stroke with a 4.21" or 4.25" crank.
Pontiac 2-bolt 326-455 blocks are strong, as far as 2-bolt blocks go. Pontiac V-8's use much larger main cap bolts than SBCs, so running them as is isn't a problem. Add ARP main studs and they're good to 600 hp. Alot of 481988 400's are drilled/tapped for 4-bolt mains, so you can add those and have the block align bored.
The Eagle stroker kits are decent- the cast crank is fine for a 450 hp cast iron head build, but if you even think you might get aluminum heads in the future you should upgrade to the forged crank. Eagle says their cast crank is as strong as the factory 455 cranks, but more than one stroker builder can tell you that isn't the case- the cast Eagle cranks have been known to fail in the 550-600 hp range.
Geno