If the crankshaft has a rear-seal flange on the end, it's most likely the old 28-oz imbalance. (The rear-seal flange is there to help the old 2-piece seal from leaking) The crankshafts from the newer engines don't have the flange, because Ford went with the 1-piece rear seal, and the flange isnt required, due to superior sealing.
I learned this when I got a 1985 302 block, which had the 1-piece rear seal, and I had a perfect reconditioned 1970s era crankshaft ready to drop in it.
I ended up having to cut off the flange or else the older crank wouldnt physically fit into the newer block. (The flange hits) I figured removing the flange was easier than finding a recon newer crank, and then trying to get the MII flexplate rebalanced for the newer crank's 50oz imbalance.
I know they switched the imbalance to 50oz sometime in 1981. However, if you get a crank that's near 1981, I'm not certain which rear-seal type it will have. So around those years, I don't know if they possibly made a few 50oz cranks with the flange or not. (or possibly made a few 28oz cranks without the flange) The era of change (1980-1982 in this case) can be tricky.
I like to only deal with 1979 and earlier, or 1982 and later engines. Those 80-81 engines are evil. haha
The part number on the block (and on all Ford parts, in fact) will begin with C for 60s, D for 70s, E for 80s, F for 90s and the second character will indicate the digit for the year. C8 is 1968, D9 is 1979, E2 is 1982, F1 is 1991 and so on)
As was mentioned previously, the 50oz cranks look thinner in the counterweight areas, and hopefully, with all the info from this thread, you'll be able to tell which is which.