I've got this devil of a project that I've been fighting with and decided not to scrap the car, just the efi. So, I'm swapping to carb.

I seem to be right on the cusp of needing one style or another and I've gotten mixed messages about what carb I should use. Some say a holley or demon 625-650 with mechanical secondaries, vacuum advsnce on the dist. and an electric choke. Some say mechanical secondaries and 750cfm....

Here are the specs.

355ci, dual plane intake, msd hei, profiler185 heads, 268xfi cam, 700r4 with 3000rpm stall, 3.23 gears.

Car is a street only car, and it won't see over 6200rpm

I have a 383 and i run a holly 750 dp mech sec, man choke. Id havent heard good things about demon. Id go with 650 holly. You may have to fart around with jets.

First off, I'm partial to the Holley platform. Whether it's actually a Holley or QFT, Demon, etc. doesn't matter too much. The style is very easy to work on and there is more than enough information available here and elsewhere to help you tune it.
As for size and type, based on the list of parts you've posted, you absolutely will not go wrong with a 600 CFM vacuum secondary version. Yours is very similar to one of my earlier builds and the 600 did the job on the street, the track and the gas station (as in getting decent if not excellent mileage). If there are fears that it's too small (which it isn't), I've had great luck with Holley's 670 Street Avenger (also vacuum secondary). I used it when I started to expand the performance of my 350.
If you really take that cam to 6200 (and it's horsepower peak is well before that), the 670 may be your better choice. Realistically though, that cam probably makes peak power around 5500 and you can size your carb choice based on that.
General rule of thumb: Engine size x (rpm of peak horsepower plus 200 rpm for overspeed) divided by 3456.