My Vitesse was advertised as being virtually roadworthy, supposedly needing only a door tread plate to get it through the mot.
Having stripped it down to the bare chassis I'm so glad I dindn't buy it with a short mot and drive it home. The chassis, which looked really solid where you could see it, was full of hidden horrors and even a minor shunt would have seen the car disintegrate around me.
The serious corrosion, however, is in the peripheral outriggers, specifically at the points where the body and front bulkhead attach. These are cheap and easy to replace and won't be taking anywhere near the amount of stress with a Sammio body bonded to them as they are subjected to with the (unbelievably heavy) Triumph body tub bolted on.
As steel gets weaker with age new metal will inevitably be stronger than a 40 or 50 year old part, so a properly repaired chassis should be at least as strong as an unrestored original. The same applies to welded repairs, so long as the welds penetrate thoroughly and the metal around them is sound they'll do the job.
The real question should be 'are classic cars safe at all?' And, much as I love classic cars, I owned enough of them when they were only seven or eight years old to know that if they were losing their structural integrity back in the seventies then they won't have got any stronger nearly forty years on without a recent, body off, no expense spared restoration costing ££,£££.
Interestingly, I bought a copy of a classic car mag special edition yesterday for an eye watering ten pounds. For that it promised me loads of tips on welding, spraying, diy trim, floorpan repairs etc. The reality is that almost every article recommended taking the job to a professional and paying to have it done. It then went on to describe how a pro car painter/welder/trimmer uses modern equipment and materials to achieve a perfect finish every time. I could have bloody well worked that out for myself. What a waste of a tenner.
The ideal solution (imho) for those of us wanting to run something 'classic' on a realistic budget is to go down the Sammio route: you get to thoroughly inspect and repair the chassis with the body off; it massively reduces the stresses placed on the weaker peripheral areas; and you end up with a pretty much rust proof vehicle that looks stunning (Gary, you're a design guru!).
So, go back to your Triumph torque and other classic foums (should that be fori?) and enlighten them to the ways of the Sammio. Give them Gary's contact details and tell them to beat a path to his workshop door with cheque in hand and they too could benefit from driving a car with a chassis that's safe enough to do the job, probably...