I would be concerned about the output too..
But you said your converter was built for a 318 ?!?! My money is on THAT being the problem, for the most part. Your new motor is driving through it.
I don't know what converter you have (who made it) but I would send it back and correct it for your new combo. Maybe $250, more if they change the stator. (probably)
If you have a stock 10 3/4 lying around, throw it in, shift it at 5800 and see how it MPH's... if it gains 4-5 mph, you've found your issue.
TrickFlows QC is probably a lot better than Edelbrock's QC... I had a OOTB E Streets in my shop a couple weeks ago, and the Valve seat run-out was horrendous!
Don't put too much stock in the Leak-Down testers.. They are finiky with piston possition and tend to show a false negative.
A compression check should suffice, as long as they're consistent, cyl to cyl.
