Operating on the assumption you are talking TR4A with IRS and that by coilover you mean adjustable spring perches fitted to the damper such that spring preload and vehicle ride height can be altered, I am not aware of one for front or rear on the TR4A/5/250/6 that can be readily purchased. Revington TR offers an rear damper kit that uses a telescopic damper with top mounts added to the frame by welding them in and lower mounts added by modifying the trailing arm and fixing them in place with fasteners. The damper then runs up through the middle of the rear spring.
It would probably easy enough to come up with an adjustable perch set for the front. On the rear probably not so easy as both the top and bottom of the springs are essentially in a well and adjusting spring perch location would require removal in all likelihood.
Here's a link to one of the Revington rear damper kits, it's one of three different versions offered. This one uses polyurethane bushings, the other two use spherical bearings in the lower mounts.
https://www.revingtontr.com/product/rtr3003spk/name/damper-conversion-kit-rear-tr4a-6
Overall, the quality of the kit is pretty good but I did not care for the fasteners supplied with the kit for attaching the lower mount plates to the trailing arm. I swapped them out for some MS24694 screws in the same diameter but don't remember which length fastener I used. This allowed the fasteners to be installed with no threads rubbing against the through holes in the trailing arm. This also required that I redo the countersinks in the mount plates since the countersink angle was different, but by the same token it minimized the chances of what we call "knife edging" bore of the hole at the bottom of the countersink.