Search the forum for other threads before you buy a HEI. It is fairly easy to get one off a later chevy/gm six in the boneyard and with some very simple grinding and the addition of a retaining collar make it work on your engine for a whole lot less money that what you'd pay to buy one that someone else modified for you.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi