Bob, that's a great offer! Nice of him to do that.
If you're talking about your 68 400 ragtop in your signature, a 750 cfm carb is NOT too big for a 400.
Do you want to stay with spreadbore configuration? Not sure which 650 your mechanic recommended, but if it's a square bore Holley design, you'll need an adapter if you have a stock intake.
Holleys are plentiful, and parts are easy to find. Both the Q-Jet and Holley will run flawlessly when setup properly. There are advantages to both, disadvantages to both. Holleys are much easier to rebuild, but because of that, they aren't as sophisticated as a Q-Jet. That being said, a Holley will get poorer gas mileage than a Q-jet.
Q-Jets used to be dirt cheap and plentiful, but since they haven't made a Q-jet in 20+ years, they aren't that cheap anymore, and parts aren't that easy to find. We've lost ALL of our pick-a-parts (junkyards) around my area, so Q-jet parts are few and far between (swap meets, E-Bay, etc.) Holley parts are actually quite plentiful. Pep Boys, Advance Auto, and Auto Zone carry the basics, like float bowl and baseplate gaskets.
If you do buy a Holley, I would opt for a vacuum secondary version, not a double pumper. If you have a fairly soft gear (3.55 or less), and you're car is heavy (yours is a ragtop), you may experience a bog with a double pumper when you stomp on the gas. That bog can be tuned out, but it takes some work. Vacuum secondaries won't bog.
I'm a Q-Jet fan myself, but as of late, I've been using Holleys. I like them both.
