All my chassis kits as well as adapter plates for mounting Sowter transformers are now available on my website www.analogvibes.com 
Thanks so much for the response, and understood!
Hopefully I've just misunderstood what I've seen above, but this quote from AusTex64
seems to say that (mounting notwithstanding) he found the Sowter's 44mm diameter to be too large. Is this not correct?
what did you guys use for the output transformer? Not many options for a 5:1 that will fit. The Sowter is a bit too big. Triads are hard to find and outrageously expensive. A-24 isn’t far behind. The Cinemag is too big too. It’s a pretty tight fit on that chassis.
I also noticed that Ron Swanson ended up fabricating a means to mount his internally instead of using the adapter plate. I assumed this may have been for a similar reason?
Because I don't have the means to fabricate an internal mount, I just want to be double sure I have a good plan. A transformer that hangs over the edge of the chassis will unfortunately not work in my application (limited rack space).
Just digging for some data, the Sowter 1290e has a diameter of 44mm, and I
think (but can't manage to verify) that the width of the HS-50 is 39mm. This isn't a massive difference, but in pictures the HS-50
seems to run right to the edge of the enclosure.
So I guess I'm asking explicitly if it has been verified that the 1290e, mounted with the adapter plate, physically does not hang over the edge of the enclosure. And if yes, is the fit reliant on
also using the Sowter 3603 (with its small 34 mm diameter), or would I be able to combine the Sowter output with a Triad input (39mm)?
I know that's a lot of questions, but I want to get it right. If there happens to be an illustration of the enclosure's rear panel with measurements, that may answer my questions as well. Thanks again for any information!