looking at the photos again I see the bike is currently wearing the second set of engine plates, and looking in the workshop I see the third set are doing duty as an engine stand
the back-story to this bike, is that in the late 60s and early 70s, there was a well-known British custom bike builder called Ray Leon. With his partner John Wallace, they built a series of converted 45s ( for the sake of availability ) using various British hardware. There was a much-photographed yellow 45 with aftermarket springers ( Leon Wallace were about the first people to import this kind of stuff to UK ) and a Triumph clutch and gearbox. He also pretty much wrote the book on these British-framed 45s and most past and current builders followed his methods. The comment about the engine mount being welded plate as opposed to milled, because of limitations of available facilities, is probably spot on. Engine plates were usually 3/16" steel in those days, or alloy from Unity Equipe and other 'Triton' builders
there were about 4 main variants;
1) Triumph swing-arm front section converted to rigid, using extended aftermarket rear section or altered stock rear section. This means the engine fitted into the original engine/gearbox space and the rear section was extended by extending the lower chain stays. Relatively simple job, two tubes to cut and weld using slugs. Geometry is less critical because any problems can be, to some extent, fixed using a cold bender!! Created room for the transmission in the 'rigid' position - behind the seat post - and effectively raked the bike at the same time, with no alteration of the main section. Usual 70s aftermarket forward controls with long brake rod! Can be further disguised using the 'long' type primary chaincase, exhaust bent to follow the lower frame rails, and aftermarket 'round' oil tank and battery tray to fill the space under the seat.
Ray himself sometimes rode a bike of this type called 'Bluegrass Special
Shoot, a man could have a good weekend in Dallas with all that stuff...