From a 'Nam vet to another of our servicemen and women....a big Hat's Off!!
Next,
from the Heart of Arkansas.
Finally, welcome to the family. That's not a bad-lookin' ride. I see that you're interested in a V8, but Stacy David did a crate engine 292 that looked sweet. Clifford Industries makes intakes and exhaust systems for 6's, and you can cam 'em up. Throw a Tremec 6-speed behind it and have some fun!
Early Classic Enterprises has disc brake conversions that allow for you to lower it, or keep the stock height. You can retain your 6-lug wheel pattern, too. If you are serious about upgrading the performance, you should consider upgrading the brakes, too. As an aside, I just looked at a '72 LWB Fleetside that can be bought for $500. It has discs and the crossmember would swap into your chassis....it's just a note that I'm sharing.
As for the grease issue, it may be the dust cap that is missing or doesn't fit well. (I had a '67 Impala and the dust cap wouldn't stay on until I replaced the hub). The grease seals on the axle stub on the back side of the wheel may also be bad.