It could be just the ripple of excitement I feel when someone mentions they have a spare set of tan leather seats, but I wonder whether you might be better off to leave the spare seat intact and get the upholsterer to repair the seat cushion foam when they re-stitch the covering. They can usually cut new foam into appropriate shapes (using something that looks like an electric carving knife). If they are really clever, they will make it out of several sections of foam with varying densities, which (knowing the 164's seats) they probably had in the first place. Soft in the centre, firmer at the edges, etc.
In my experience of other seats (not the 164's) the seat covering is usually attached to rods embedded in the foam, making it a surprisingly complex operation to remove and refit the covering (the little triangular 'hog rings' are difficult when you're working within a thin slot in the foam without the correct pliers). If there are wrinkles, you'll be forever annoyed, and (like you find when you try to repair a shoe), it's amazing what a hammering the seat base takes - something that looks nice to start off can quickly turn baggy and crooked.
Of course you can do it, but it's one job that I place in the same category as windscreen removal/refitting - something I'd rather leave to someone who has the correct tools and does this often.
I had the rear seat back of my old '89 164 repaired (the stitching had rotted in the sun, so the top of the backrest had two big splits) and I was amazed by the quality of the work I got for NZ$50 (about US$35) - they restitched most of the top of the seat, just as evenly than the original. I can't sew in curves like that (really, I can't sew to save myself) and I only wish I'd decided to have them use new un-faded material.
'07 CLS 550, '09 500 Lounge Dualogic, '71 850 Coupe, '03 Stilo Abarth Selespeed