The GTV has a solid rear axle. The suspension is called a DeDion design(ithink).
More knowlegable people can chime in, but it is essentially held by locating
arms with springs and shocks at each end, and flex is controlled via a trunion.
A lot of racers use a sliding block rear end. If you want to read more about
that, do a search in the suspension section of the BB.
BTW, you might want to look closer at my welds before you have me work on
your 510
