I just measured mine. I have 1.5" measured just below the head - horizontal from the block to the vertical part of the firewall.
Then measuring just below the accel. linkage bracket horizontal from the block to the curving part of the firewall I have 2.5".
Now, I will add that I may have a tad more clearance than is typical because I know that where some guys are unable to reach the two top bell housing bolts, I can get to mine without too much trouble - not easy, but doable.
I'll just throw this out there for the heck of it...are you certain that the accel. bracket and linkage arm are all in the correct orientation? The angled edge of the BRACKET should be mounted up, and the flat section of the LEVER is mounted in the down position connecting to the pedal while the round section mounts up and goes to the carb. linkage. Of course, you would be the first guy to have ever made this mistake!