Post a pic of the amp head or email me a picture and I could tell you for sure which one it is. PM me for my email address if you want.
Aside from that, I think I would try it and see. KT88's won't make a whole lot more power than EL34's in that circuit because the voltage isn't changing, and the output transformer's impedance ratio isn't changing, so the only difference is the tubes themselves. I would say you are probably ok.....if a Silvertone 1484 output transformer can handle it's operating environment then the Sunn transformer cna handle it with KT88's. The Silvertone 1484 uses two 6L6GC's with 475v on the plates and 340v on the screens and an output transformer the size of a champ, where it should be the size of a Bassman. Now those transformers do blow, but they also work for many years. The Sunn 40W OT is bigger proportionally so I don't think it will be a problem. Remember that these Sunns were basically Dynaco hi-fi copies with a different preamp, and hi-fi transformers are larger than they need to be for guitar amp duties. Bass amps benefit from hi-fi transformers but its not common to use them, and Sunn was the main one that did. So bottom line is that they are oversized comparably already so it should be fine.
Greg