before i give you an idea how to do this, let me say. you are dealing with a convertable and center body weight could hamper ride, performance, and body integrity. i personally would look for seats with correct bolt patterns.
i assume you have an overlap(seat runners on new seat are longer than originals)
1st, check the width of the new seats to be sure they will fit between the hump and door.
cut a piece of plywood to fit between the hump and the inside of door opening and overhanging the original seat mounts by 4 inches front and back (plywood should be thick enough not to bend when ya put seat on it).
mount plywood in place of current seat, with equal overlap front and back (ya might have to counter sink bolts to allow seat runners to set on wood)
staple or tape a piece of corrugated cardboard to plywood.(cardboard should same size as plywood)
set seat where ya want to mount it on top of cardboard, and mark mounting holes.(also mark original mounting bolts from plywood).
mark cardboard with a felt tip marker as to side
(driver or passenger)this is your template.
remove cardboard and save.
repeat on other side, using a fresh piece of cardboard.
take your templates to your local steel works and have them cut the pieces you need (3/16 with minor overlap, 1/4 for major overlap)complete with holes.
mount steel plates to car, then seats to plate.

now, i still urge you to find seats with the same bolt pattern.
at the very least, install a set of x-braces for stability.