I think you will find that the mk2 recaro seats or even a mk2 16v rainbow are the same set-up as your seats. Welding won't help, it will give way again, I've seen this happen too many times...
do a search for mk2 16v seats for sale and get picture of the bottom of the seats from seller to confirm set-up and then buy the worst seat possible with a great frame and swop them over...