Just a bit of advice...I would check your front universal joint angle so that you will be able to move the tranny tailshaft up if you need to before you finalize your tunnel. I installed a 5 speed in my 68 b-body and checked my front operating angle and it is slightly pointing down instead of upward. I can't raise it up anymore in the rear because of the tunnel.

Your should strive to have equal and opposite angles from the rear end /driveshaft angle to cancel out any viabrations. The rear usually points down about 3-7 degrees, so the tranny should be pointing up about the same.