Re: what carb for a 390 fe
A 650 cfm is ok, but keep in mind, if your going to use the vehicle for cruising, daily driver, work and back, low rpm's...etc...a 400 to 450 cfm would be perfect. A 650 or more is good if your wanting to get maximum power. ie: running hard, showing off...this sort of thing. a 650 will work, but a smaller carb will be more efficient and easier to keep tuned.
It all depends on what your trying to get out of the motor....performance or effiency.
catskinners carb calculator is is good....80% is stock, 81% for what you have, 3500 to 4000 rpm ...normal driving. 6000 rpm if you run it hard.