What will it run?

You're right about the 8.1cr. Not a lot of low-speed torque at that number.
I think the problem is in the plenum size. When you drop the hammer, the manifold vacuum drops like a stone, the airflow momentarily stalls.
A Small-port Dualplane,with a small plenum, will keep the vacuum up, and prevent the boosters from stalling, and will pull up the low speed torque.

87mph is pretty good; especially so with a passenger.