Hi, I haven't ordered mine yet but you might try the search feature to see if it's been discussed, I know it has on other classic boards. Below are some of the options I'm looking at for my '67 Coupe when I get that far, I like to keep the original look so I found plain SS looking belts, all metal, I didn't want that '80 plastic looking buckles. I think GoBelts was the one I decided to go with as I've got more links to them, emailed a while back and they replied everytime w/in 24hrs, good customer service so far even though I haven't purchased yet.
I am still deciding between using a retractor at the bottom or just plain set it and let it hange. I think you run into problems with the seat hitting the retractor in the original location for the lap belts, some people redrilled and moved it back towards the rear seat. You will need to get a longer sash so it comes down from the roof and lands above your shoulder, they install that when you order it, just let them know the length, I think the 8" is what I decided lined up best.
Plenty of reading for you,
Website showing the install of Hyundai belts in a '67 Coupe front and rear, gives you a good idea on mounting points, we're lucky as there are provisions for this under the headliner:
You don't HAVE to go with aftermarket seats but I am, picked up a pair of '90GT seats with headrests, being 6'+ the stock seats end below my shoulder blades, not a lot of support. While the 3 point belts will keep you from hitting the steering wheel the whiplash could break your neck so I figured newer seats were much safer, more comfortable for long drives, fully reclinable, mine have lumbar adjustments, hug you in the seat better, etc. I will be ordering seat covers from TMI/Laurel Mtn. that make them match your rear seats so they look factory for ~<$300/pr. I can always install the seats and buy them later, not hard to install, no hogrings to deal with.
PM me if you need more info or other forums that I've found info. on.
Good luck, Jon