James, I can't tell you the difference between the Delo 100 and 400. I suggest you print the most recent two cycle oil recommendations from MTU and provide them to your oil distributor, with the applicable info let him help you make the decision.
Several years ago the key to finding a suitable Detroit two cycle oil was it had to be "low ash" but today with the new diesel particulate filters on diesels you'll find most all oil for diesel engines is low ash, which is nice for us Detroit folks, it's easier to find the right oil now.
Again I'd run straight weight oil unless Detroit currently and specifically tells you otherwise, it is what they have most always and consistently advised us to use.
A 4-53T in a 78-80 or so GM truck is not that rare, in fact that's where I got my first 4-53T. Prices on the whole trucks are usually less than the engine alone is worth, so a good source for an engine if you can disassemble and part out or scrap a truck.
