Do not rule out rear main seal...
To clarify, the rear main won't be the cause of the oil pictured on the block... but it could easily be the cause of a slipping clutch.
The external leaks are not a likely cause of your slipping clutch.
There is only one sure way to know why the clutch is slipping... pull the trans to examine it. There are really only 5 "likely" causes: Clutch cable out of adjustment, worn out/glazed clutch/flywheel, weak pressure plate, leaking trans seal, leaking rear main. Only the cable adjustment can be confirmed 100% without pulling the trans.
Replace the distributor o rings (2 on Carbed, maybe only 1 on SI) and valve cover gasket. Those are really the only likely things to leak up there, and these parts are cheap. (While valve cover is off, I'd adjust valve lash) Also, change the O-rings even if dizzy isn't leaking; they are old and getting ready to leak.
Since you say you're pulling the trans, take off the flywheel and examine the rear main seal; make sure it isn't leaking. Check the trans side seal too. If either one of these is still leaking when you install the new clutch, chances are it will be slipping again within days. (ask me how I know >_<) I'd recommend having the flywheel machined and balanced, but many have gone without and had no problems.
Where can we get distributor o rings? I've searched the forum to no avail. (sorry for the somewhat off topic post, but may be helpful in this case)
Any automotive parts store will be able to match them up. You could also buy one of those assorted O-ring packs, but you'll have a lot of spare O-rings...