I would even venture to say it would be impossible. Unless you have a very good engineering qualification and can certify the resulting strength and flexibility of your work to the standards to satisfy the examiner or can find somebody who can, it will never pass an IVA and it shouldn't pass an MOT either unless you completely reconstruct the connection which is almost impossible on a Triumph chassis. We aren't talking about a rust patch repairer here, which may be acceptable for the MOT depending on where it is (some parts of the chassis can't be repaired, I am told).
I have installed the Ford Transit chassis extending kits on their pickups and it's no minor procedure. They are plated with sandwich plates, inside and out and all bolted, 10mm bolts into captive nuts attached to the inner plates, welding is NOT allowed for the reasons Micky has mentioned above.
Modding the body on the other hand would be a doddle and nobody (in authority) will question it.
Good luck with the build!
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