I've done this conversion many times. Using the tried and true 283 and last time the 350. The cast bell housing from 60s and 70s pickup is easiest three bolt starter Rams horn manifold. Last truck I did I used set of headers from Summit instead of Rams horn header and it worked with more clearance to steering box. So check out Summit for the block hugger headers. They come off head and go straight down and in so plenty of clearance

58 gmc 100 with 350 sbc