Yeah, I'd guess that the oil pan won't support the weight of the engine -- not even enough to change one mount at a time.
There usually are lifting eyes bolted to two diagonally opposite corners of the cylinder head. The engine can be hoisted with these. There is a removable section under the rear bench inside the bus which allows access to the rear lifting eye.
Devising a scheme to make use of the lifting eyes might be problematic, however.. maybe lifting by the front eye alone will be sufficient to get the weight off your mount.
Failing that you could remove the oil pan and jack against the bottom of the block, ie the oil pan mating surface. Use a block of hardwood to protect the surface and take care not to jack against parts like the crank shaft, crank bearing caps, etc.
Dry weight of the ISC 8.3 is published about 1630 pounds, but that's probably without manifolds or accessories and definitely without turbo, exhaust, and a bit of transmission weight too. A person would change only one mount at a time and some weight could be borne by the other mounts, but if you prepare to support at least 2000 pounds you should be in good shape.