Big Block seems to slow

Around 35 years ago a guy I used to work with bought a mid-70’s polara that had a well used 400 in it.
Someone had put some oddball aluminum intake on it(don’t recall what, but I hadn’t seen one before that), and a Holley 4bbl.
They had convertered the car to points, and it was running like crap, popping thru the carb, etc.
They thought it needed a timing chain.
They didn’t want to fix it so he bought it for dirt cheap money.

I told him I thought it acted like a bad condenser.......so, a few dollars later and a new condenser and it was running fine.
He drags a somewhat beat up Duster home from the junk yard, and puts the drivetrain from the polara in it.
It had a single dig 2.76, the 400, and there were some rusty fenderwell headers in the trunk of the Duster, with holes already cut out for them...... so they were installed.
The 4bbl Holley ended up being a 390cfm unit.

It went mid-14’s like that, if you walked it off the line.
If you just mashed it, the one tire would spin almost forever.
I don’t remember the speed, but I don’t think it was as good as the OP’s 361.