Hmm, just re-read this lot, and hope you have a 200 cube engine. I've assumed you have!
Yes, with the right secondary metering block, a 390 will get pretty close. Stock ones were rated around 155 or so hp, and there is another size up if needed.
You need a manifold which allows the secondary ciruit to breathe into the log manifold, if its a 200.
I've been thinking of late that one may be a very tight fit on a 200, and the adaptor would have to be a little like the 2.6 or 2.8 Mustang Cologne V6 number which mounted the 5200 Holley Weber or 32/36 Weber 2-bbl. It had odd porting, but worked well.
Here is the stock sequential Holley Weber 5200 to Solex 34 unstaged European 2600 manifold used in early Capris.
Below is the Schendahl Bro's 2-bbl Autolite 2100/2150/Holley 2300 modification to the big 1.75" intake log six.
The idea is to have a similar adaptor which is just two wide rather than one throtle wide.
200stang had a similar idea a while back, but we thought the porting wouldn't be ideal as he was thinking of a 600 cfm carb from a 5.0 GT Mustang. A 390 carb, it might be okay.
Just remember, the carb would have to avoid the hood, spring tower and rocker cover. A 200 manifold is closer to the spring tower than the 250, but the 200 is shallower, and can have a tall adaptor without taking the hood out as its 1.7" lower than the 250.
