273 or 225?

For me, the sound of the exhaust would be worth the swap.

My Dart slant sounds like a Ferrari with the Dutra Duals and accurate exhaust. It draws a crowd. I have a 66 with a 360 backdated to look like a 273 and I still enjoy this one better. Deck the block to get compression, oversized valves, Dutra Duals or headers, recurve the distributor and more carb and a 225 will make you smile.