Laurie and I did the seats in our GT-150, and we lived through it.
made patterns from the old seats
sewed one together from a table cloth as a trial run
used pre made piping and matched the seam allowance to it
stapled it together then sewed it on a singer we got at a thrift store (you need to use a zipper-foot)
we found the piping and pleated vinyl here
http://www.garysupholstery.com/i made new wood for the back-to-back seats for the 88 X-19 i am restoring but have decided to go with swivel buckets, bench, sun-pad.
you can get them for what i have into the plywood and tee nuts