The pooling around one injector may indicate a leaking injector body. You can disassemble the injector, clean the crap out of it, and reassemble to 55 ft-lbs of torque (head to injector body).
If you're getting a puddle of fuel, it likely is the o-ring between the distributor head (cast iron part) and the aluminum body. Only that seal will cause serious puddling of fuel. Either way, you need a complete reseal (or ideally, rebuild).
User name "red---rider" on ebay used to do reseals for about $150 shipped both ways. But I'm not sure he's doing it anymore. I contacted him a few months back and he said he would still do them, but had been traveling a lot. You can use the "advance search" function to "search by seller" and find him. He's in NC.
There's also a shop in Memphis that does rebuilds and advertises on ebay. $375 for a rebuild ain't bad at all. Though, there may be delays due to the flooding, so I'd call first. I've used the reseal guy, and a friend had his TD pump done at the place in Memphis. No complaints on either end.
$500 for just a reseal is a ripoff. As noted above, you can get a complete rebuild for less.
The pump off the 1981 will work, but you're sure to be in the same situation in a few weeks/months, unless that used pump has been resealed. It's not a question of "IF", but rather "WHEN" an old pump will start leaking. I thought my 1984 pump had avoided it. I sold the car about 3-4 year ago and have done maintenance on it for the guy who bought it. It lasted about 4 years on the new ULSD before it gave up. I'm resealing it right now.
Speaking of which, if you wanted to do split in costs, I have a pump I "rebuilt" and resealed. It hasn't been calibrated like a real shop would do, but it was a running pump that I completely disassembled, cleaned internally, and resealed. I've done quite a few with great success. I can't guarantee MPG though, since they haven't been calibrated to the new fuel.