The issue there is there isn't a standard, since coils weren't all that common to use in a bucket. You'll want to find some from a very light weight application, and also the same diameter as your current ones, unless you're willing to modify the upper and lower spring mounts. Also they can't be "pig'd" (where the ends curl in to get bolted into place). Corvair fronts seem like a good suggestion, a light car and rear engined, if they have the right diameter, that'd be a good starting place. Ideally your new rear springs should squish down a little bit when you climb into the car, then you've got your spring rate matched to the weight of the car well, and it'll have some give when you go over bumps and such. This will probably involve some experimentation and lots of measurements. A spring compressor would be a good investment. Luckily the way it's set up, they'll be super easy to change out.
A T generally needs a 150-175 lb. rear spring rate .IDK , but someone like Eaton might be able to furnish you w/ a custom wound spring in the length you need w/that spring rate ... dave