A 2004R or even a 700R4 for it's 3.06 first gear along with a adapter plate and 3.42 and up gearing will make a big difference. A 390 Holley or 500 cfm AVS2 with tuning or the best bet, a Buick special 4.1 Qjet, betting it is restricted in cfm. A 600 cfm is way to big for a 3.8, it would need a ton of tuning and still be too much. I would have transferred the Duajet's primary metering rods and APT position over to my a Qjet, along restricting the secondary air door and playing with secondary rods on my 260 if I had a SP2P intake still. The 4.1 Qjet might be slightly richer, which is good if adding headers and dual exhaust, which will also help. The headers and duals felt like 20 hp/tq on my 260, especially up top. With the 4bbl, it would be in vin Y 307 territory with slightly better mileage, in theory. Just expect a small improvement, a 8 to 1 small ci and cam motor can only do so much. Pushing in too much air and fuel will overwhelm it and hurt performance.