Originally Posted by B-string
Have you tried http://www.magneticcomponents.net/
? I just installed one of their OTs in a Marshall 6100. It turned out to be not only a good replacement but a tone improvement as well. Creamy OD, more open lows and sweeter highs.
A "SVT' or 400/400+ OT would be a special order (300 watt plus) but they do special orders all the time and more reasonable than MM?
They do indeed make great stuff; I have used them for guitar as well. I inquired about an SVT style a few years ago and they wouldn't do it. Heyboer are great and very well priced. I've got five custom chassis waiting for a big amp, and the design is done, but as Bassmanpaul said, it's a huge undertaking.
To the OP, if you haven't built an amp before - make that several - DON'T start with something this complex and dangerous. .