Another update: after getting the bulk of the external painting and metal work repair done, we took the tank for a drive around the arena on a very hot and dusty day. (There are separate threads showing this, I think.)
Two problems showed up: the old oil seals in the transmission and the final drives leaked, and the engine ran hot. Not dangerously hot, but at the upper range of comfort.
I confess the oil leaks from the lip seals on the transmission input shaft, the output shafts (4 seals in total), and the final drive input shafts weren't all that surprising since the tank had been gate-guarded in the late 70's, so the seals were at least 40 years old. I took a flyer because getting the transmission back out isn't a lot of work, and I knew finding replacement seals would be tough.
The overheating problem is most likely because the fins in the one rad are still clogged with an oil/dust mix. We spent hours with varsol and pressure washing trying to clean the fins and hoped for the best, but it it seems we need to do more. These are very thick 8-row rads so crud getting deep into the core is very hard to get out. I'm thinking steam cleaning now.
When's it's running again, I'll take infrared readings of the inlet and outlet temperatures of the two rads to confirm this is the problem.
Regarding the seals, we needed to find seven new seals. I found three seals of the correct size, but the other four were a problem. If the inside diameter fitted the shaft, the outside diameter would be too small. So I ended up getting the seals that fitted on the shaft and buying some mechanical tubing and machining spacer rings to make the seals fit in the housings. I'll post some pics and the seal part numbers for the tiny number of people who care.
Malcolm