The 'goo' on the floor was there for two reasons. The first was rust prevention and the other was sound damping.

The rust proofing part did not really work out as the goo developed cracks that allowed moisture to reach the metal where it was then trapped. So the floors rust out.

The sound damping part works, but there are better ways to do this today than there were 50 years ago. Truck and Van accessory shops sell a sound damping, rubber type of blanket that needs to be applied to the floor of a truck or van to do the same thing the goo did: stop the metal from vibrating as you drive.

If you remove all the black goo you can then put down this blanket which will not crack. Well, maybe it will 50 years from now.

Your rear axle has a spring compensator and not a hydro-compensator which is a good thing since the hydro units are very expensive.

I agree that the vent hole on the top of the rear end should be cleaned out. Check your rear end fluid level while you are in there since it appears that a lot of it has leaked out.