4bbl intake for 290 - cheapest?

Stock 4 Bbl iron intakes from ‘66 to ‘69 only accept the Carter AFB as far as I know. The Carter AFB is a good carb and came standard on ‘68 & ‘69 AMX w/ 290. If you get the 390 casting # 3191736 there is a slight 10HP boost. 

The early version of the R4B can be used on your engine w/o modification and w/ a Holley 4 Bbl - a bit pricey w/ AMC p/n, not too bad pricewise without. A new or refurbished 500 CFM Holley carburetor w/ electric choke & vacuum secondaries from Summit on a street 290 would be nice. 

Right now I am trying to rebuild a Rochester quadrajet to go on the 350 that will go into my truck. At 750 cfm I think that one would be a bit much for the 290, although they did also use them on 307's. 

Even if you don't race, that will sure help with pulling out into traffic or passing someone. MPG is better as well, along with throttle response. Much crisper.

Since you're in the UK what about a Weber 38 DGES? Lots of Jeep kits made for hi-po 258s, so should work well on a stock 290. The Jeep 258 adapter should work with the stock 290/2V intake -- AMC sixes used the Carter BBD which has the small bolt pattern, not the Holley standard that later model V-8s used. I think it's the same as the carbs you have.

You can get 2V to 4V carb adapters that will work, but they do restrict flow some at higher speeds. At normal highway speeds you will rarely trip the secondaries in, but when you do they won't flow well if opened very far with the adapter. I'd stick with a 450-500 cfm carb, but the 600 can be tuned to work well. The secondaries just won't open as fast or far as a smaller carb. Generally, the smaller the primaries the better power at low speeds though. You give up some high speed power with the smaller carb, a little low speed with the bigger. That's why for pure street use a smaller is generally better.