[kwmech]

As mentioned the big block stands are shaped different and when bolted in the same place as the small block stands they actually move the engine rearward 1.25'' and they allow room for the cast iron exhaust manifolds. The big block cushion is thicker than the small block and has an additional cushion underneath which interlocks with a 1/2 bolt to keep the engine in place under extreme acceleration. You can bolt the big block on the existing stands, but watch out for fan clearance at the radiator with either water pump set-up you use and may have to go to electrics.

All in all, spend a little extra and get the correct parts, you'll be happier in the long run and it'll make for a nicer conversion