The Edcor XPWR011-120 power transformer and the Edcor CXPP60-MS-4.2k output transformer are electrically almost the same as the Dynaco P-782 power and Dynaco A-431 output transformer. You are correct in the the 4200 ohm vs. the 4300 ohm difference on the Edcor and Dynaco output transformers is negligible. However > Neither the Edcor CXPP60-MS-4.2k output transformer nor the Edcor XPWR011-120 will physically fit the bolt hole pattern on the Dynaco Mark III chassis. The only way you could use these transformers is to make up a Mark III is to (A) build your own chassis OR (B) drill holes on the stock chassis to make them fit. The other thing to think about is that any "custom build" will always be looked upon with suspicion and be worth less should you ever decide to sell your amps some time down the road. In other words, an original looking Dynaco Mark III or a build of a brand new Mark III will always be worth more than than a custom version Mark III built on an "alien" chassis. Also, when you buy parts piecemeal (driver board, tube sockets, quad cap, choke etc.) it will cost you more for the parts than if you purchase a complete kit.
If you are looking to build a pair of Mark III's my recommendation is to get two Dynaco Mark III kits from Dynakitparts. When you buy a full kit, the price of each item is somewhat discounted and the total price you will pay is less. When you are finished, the amps will look just like an original Mark III and have greater value if you do decide to sell them at some time.
Last edited by Bob Latino on Fri Aug 19, 2011 1:18 pm; edited 1 time in total