magnums may require modification to the accessory drive, specifically the alternator mount bracket and possibly the PS bracket. the accessory holes are in the same location as LA heads, but all on the same plane, wheras on an LA the lower, outer hole is rearward like 3/4"....

magnums do make a fair amount more power stock, and magnum 318's get pretty good mileage thanks to thinner rings and roller cam. and with a mild cam regrind make some good power.

how quick and how cheap do you want to do it? if you can find a running magnum, I'd go that route. replace the valve springs with hughes #1110 and their retainers, and send the cam to bullet cams to get it reground with their HR259/316 lobe, reuse the stock lifters. get a professional products 4bbl intake for magnum heads.

another option would be to find an '85-up M body (5th ave) engine. they will have the closed chambered 302 heads and a roller cam. should be able to find a good running one for under $400. do the same cam regrind/spring swap and an eddie performer, and it should perform similar to the magnum up to about 3500 RPM, where the superior head flow of the magnum will allow it to pull away.