im thinking some of the bbc truck blocks had thick cylinder walls.3999289 has one of the thickest ......
http://cgi.ebay.com/ebaymotors/CHEVY...spagenameZWDVW ....i believe some of the truck blocks can be bored to 4.375 safely!!!something like that if your on a budget.if your gonna keep around for long time and can afford it id recommend an aftermarket block,then you dont take the chance of a used block being cracked,or otherwise someone elses junk!if you shoot for 700 hp as a goal in a 496 or larger motor you shouldnt have many problems...i remember reading about 496 combos making 700 hp on premium pump gas like back around 2002.with a 300 shot of nitrous whoaaaa1000 hp yee hawww!!!bbc stuff is getting so reasonably priced now a days!im thinking you can get a unassembled dart shortblock 540 for about $3500.check ebay or racingjunk.the bigger the motor the easier it will be to make power!torque is what pushes a car down the road(if you can hooker up!!!)cylinder heads are also reasonably priced nowadays,if i was to do it again id buy a cast head then take to engine builder for porting and assembly of good matched components.good luck!!!remember bigger is definatly better!!!