Given your initial information a small block 12 or 15 vs a big block 15 you will not find a small block that will come anywhere near a bigblock 15. Because small block 15's get there with bore increase = rpm and big blocks get there with stroke = torque. To push a 235 car effectively you need stroke which = torque because of its heavier weight. It is not possible to gear a 235 low enough to effectivley use a high reving 15 small block. Been there done that.