With an aftermarket block(preferably a Dart Machinery) you can more than double how far out you can bore them. They have a lot more material(#1reason for strength) in them. If you are just going to do a stock block I'd try and stay .030-.040 over but you can go .060 over. Have it machined for a stroker crank(331-347) with aftermarket pistons,rods,etc. or you can build a 306(.030 overbore w/stock stroke crank) A 306 build can reuse your rods and crank(some reconditioning is recommended ie turn/polish the journals,arp bolts,etc) With good heads,cam,etc you can have a nice stout street engine that can last a long time(350+rwhp) and a stock block 331-347 built properly can make 400rwhp or more