One of the greatest fallacies that still pervades today is that these older "cloth lead" Dynaco transformers are somehow "better" than anything produced today. What they really mean is that, in the collection of mediocre (by today's standards) parts that the Dynaco amps were made up of, the only real thing you should use in a complete rebuild are the OUTPUT transformers. The output transformers were very good and well made. BUT - these original Dynaco output transformers have been reverse engineered by a number of companies here in the USA. It isn't hard to take an output transformer apart and note the core size, wire size and winding technique and replicate that transformer. I did a listening test a little over a year ago with two audio friends with two identical Dynaco ST-70's. One had the original cloth lead output transformers and one had new production output transformers. Try as they might they could not tell any real difference in sound between the two amps.
It makes little sense IMHO to buy an old Mark III off Ebay just to get the output transformer. Depending on condition one Mark III will go for $250 - $400. Besides the (probably) good output transformer what else do you get? A power transformer that could date all the way back to 1957 when the Mark III was first made. The power transformer is the "engine" of your amp. Do you want a '57 Chevy engine in your new car - a car you are going to drive every day? A driver board made of an inexpensive phenolic material with open solder traces. Very cheaply made input and output terminals. A nickel plated chassis that is probably starting to show signs of rust and corrosion. A woefully undersized quad cap with not really enough power storage to run the amp at high volume levels. A set of tubes with who knows how many hours on them ..
If you intend to play an amp like this every day it makes more sense to start new. For the $325 you were going to pay for the old Mark III you could get a NEW power transformer, a NEW output transformer and a NEW stainless steel chassis that won't rust. Just add tubes, a driver board, input/output jacks and a quad cap and you have an amp that will look better and be more reliable than any original Mark III.