An L78 400 uses a no. 500557 400 block- it's too weak to handle a 4.21" or a 4.25" stroker kit. The main cap saddles on a 500557 are too thin, and typically crack at the dowels that locate the main caps. Look for a 71-75 no. 481988 400- these are much stronger and thicker in the main saddle area, and can handle 650-700 hp, even with 2-bolt caps. A 455 is also a good alternative foundation for a high performance build- 71-74 no 485428 blocks are good to use, as well as earlier 70 blocks.
Once you have a 489188 block, you can add a 4.21", 4.25", or even 4.5" stroker kit. A 455 can also be used- if you don't plan to go over 500-550 hp, the stock nodular crank is more than adequate. Butler Performance is also not the only source for Pontiac internals. KRE, SD Performance, Sandoval Performance, CVMS, Pontiac Dude, DCI Motorsports are all well known high performance Pontiac builders. All sell rotating kits, ported heads, cams, too, so check with them before you buy anything. Keep in mind these builders aren't cheap either, so prepare to spend some money if you want big hp.
Geno
