Your logic about the spacer would seem right in theory. I've played with that intake in a big block mopar on the dyno. I tried several spacers. Best one was to use an HVH super sucker 1" spacer. It makes 10-15hp and similar tq.
I run 3" car seems to like it and it's quieter and deeper sounding. If you are going to spend money on an exhaust just go 3". If you already have the exhaust a 2.5" would be fine for your combo.
