Upgrade for '70 Swinger 340 rear

how much do you want to spend, what bolt pattern do you want to use, and what type of gearing do you want? if you want the large bolt patter, on a budget, your best bet is to get an explorer 8.8. Older ones use 10x2.5" drums, newer ones disc. Around here you can find 3.73/Limited Slips in yards everywhere for less than $200, grab a 2nd short axle out of another one, shorten one side of the housing to center the pumpkin (using the 2nd short axle shaft on that side), cut off extraneous bracketry, and weld on spring perches. That gets you a rear about 56.5" wide that will handle a ton of HP for not a lot of coin.