I've thought about doing this on a future project if it'll work. The goal- make 700+ horsepower with a production casting smallblock. A friend of mine has been running 10's in the quarter with a 377 combo, he started cracking stock production 400 blocks at the 600hp level and ended up going to a Dart block. The cylinders were cracking on the factory blocks. I was thinking of sleeving a 400 block down to 350 bore size to increase cylinder wall thickness. Then do the buildup with a 4340 3.75 stroke crank, h-beam rods, good pistons, studs throughout, possibly billet caps, 1/2 block fill, mild 9.0 to 1 compression, streetable nitrous grind cam. Would this combo handle a 400to500 shot of spray? Could be a really streetable engine with plenty of power when needed. The cost of sleeving 8 cylinders could be expensive but I have a good machine shop connection that'll cut me a deal if the work was done as filler work in the shop. If the engine made 300hp off the bottle and was capable of withstanding a 400-500hp hit it sure would make some big numbers. I read an article one of the mags did a while back where they threw nitrous at a stock smogger 350 until it popped, the thing handled like 400 in stock form! Any ideas? What would happen? :D

In the old days, sleeving all 8 cylinders was done on race engines to strengthen blocks...local guy has a ex Bob Glidden 351C block that is done that way.

With todays good strong aftermarket blocks, why would you want to bother with doing all that machine work and still not have as strong a block as what the aftermarket offers?

I can get the work done for almost free.

And I already have a block. They don't give aftermarket blocks away.

I still think you'd be better served in the long run by using a Dart block or other aftermarket block.
Even if you sleeve the 400, it still doesn't have the thick deck, blind tapped head bolt holes or extra thick main webbing of the aftermarket block. Nor does it have the priority oliing system that oils the mains first.
High cylinder pressures will find ALL the other weak spots in that 400 block besides the cyl. walls.
There's lots of reasons why the aftermarket block is better in a high-HP nitrous application. I'd do the aftermarket block in a heartbeat. JMHO

Randy