With a 1 inch clearance between the carb and the firewall (so the carb can breath) the total length of my engine is 8 1/4inches from firewall to the prop nut face.

Now, my conversion uses a prop nut that may be slightly longer than other aftermarket units as it was made on a friends lathe (very nicely done). If you are going electronic ignition (no flywheel) you can probably get the engine down to 7inches long without a problem.