Hey LBiR2
On my BBC with a "Blower Shop" set-up
I measured 32 inches, maybe 32 & 1/4"
That was from Flexplate to front of lower crank blower pulley.
I did not include bellhousing cover in that measurement.
Or...
The front of the pulley rides out about 8 & 1/4"
from the block itself . (where motor mounts would bolt to block)
Keep in mind that you may also be wanting to add a blower
"belt guard shroud" to protect hair, fingers etc. for
rear seat riders. That would add another inch or so
in front of the pulleys.
I'd mount the seat after the motor is installed if you're
wanting to get it exact.
So... whatcha buildin'???