How much clearence do have between the floorpan and collector flange? Also the TTI instructions should have the measurments for the engine position.

I have the same set-up as you RB with a 5 speed but in an e-body. I had to do some tweaking to get extra clearence since my car sits pretty low. The spacer helped quite a bit on my car. Don't understand how you could have the same clearence when you put the spacer in.