The Offy 6 intakes that I have seen are set up for a square-bore carb.
The Q-Jet is a spread-bore, thus the need for the adapter as mentioned by others in this thread.
Carter used to make an aftermarket AFB that was specifically calibrated for a 283 Chevy in a 400 CFM size. Two were made: part number 9400s (with no EGR provision), and 9410s (with EGR provision). I had a 9400s on a 300 CID 6 for over 400k miles with excellent results on our shop truck, so personally know these things will work. Believe it or not, the 4-barrel Carter got almost DOUBLE the fuel economy of the 1-barrel Holley it replaced.
The big issue is price. No, they were never used as original equipment, and you probably have a better chance of finding a needle in a Kansas hayfield than finding one in a junkyard.
They do occasionally surface on Ebay, but never cheap (if a decent core).
But the Q-Jet for the Pontiac 250 is a collectors item, and not inexpensive either.
Generally speaking, the cost of rebuilding the Carter is much less than rebuilding the Q-Jet. 90 plus percent of the pre-1975 Q-Jets are going to require both primary throttle bushing, and a heli-coil in the fuel inlet. While hobbyists try these operations, generally the hobbyist fails and ruins the carb. Both procedures should be done by a competant machine shop (or a hobbyist who has access to, and can operate a vertical milling machine). I have yet to see either procedure required on any AFB.
Another possibility would be a Rochester 4-Jet (the 4-barrel from Rochester prior to the Q-Jet) for a 283 Chevy. They are square bore, and approximately 400 CFM. Also, they have a cast iron throttle body (unlike the aluminum Q-Jet T-body) and virtually never need bushing. The fuel inlet also rarely needs attention.
Since most enthusiasts would rather have the Carter WCFB over the Rochester 4-Jet on the older 283's, you still have a ghost of a chance of finding a rebuildable core in an older yard.
Jon.
