Search the posts in this forum for info about selecting the right rear axel gearing and figuring out the width of the axel you'll need(from tire mount to tire mount).

Basically, you've got to figure out what bolt pattern you need, know the gear ratios in your tranny and then do some math/calculations using an online calculator to arrive at the gearing that'll work for your needs. Then measure the rear axel and tire rim backspacing to get the width axel, then go looking in the junk yards for one that'll work. Finally, you'll have to cut/grind the leaf spring/coil spring mounts, get a set of spring perches (about $12 for the set), figure out where to put them on the axel and have them welded in.

Don't forget that swapping the rear end means getting at least a new u-joint for your driveshaft to match the pinion on the new axel.

Finally, another option would be for you to change the ring and pinion gears in your existing axel. It is sort of a pain in the @$$ but not too expensive to have someone else do for you if you take them your axel and the new gears. Depending on the diameter of the ring gear in your axel you might be able to get new gears for about $150 and have them set up for about $250.

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