No, did I say it was? Have you patched a floor of a 240? The metal isnt thick, probably thicker then a hood per say, but when replacing a section of the floor with metal a couple thousanths of an inch thinner, do you really think that will sacrifice the intgrety of the floor? If he was worried about the structural aspect THAT much he wouldnt be putting his time into this car and would be welding instead of riveting.
This will fix the problem, taking into consideration the tools, time and skills that he has, and Its a cheap alternative to spending possibly close to $100 on metal, if he pays the price you did.