Hey there, I was planning to install those basic safety features for a long time but it had lots of interdependent things like bucket seats need to be secured and brackets welded back (I know on the images the brackets are still not there but I have a welder coming tomorrow).

I got the 3 point retractable belts from Wesco Performance (they were around 350CAD  with all deliveries and fees) and had a pair of lap belts from before. There was not much issues drilling holes for the rear ones and screwing bolts. 

For the front ones 2 more big holes needed to be drilled per side, one on the top and one on the bottom for the retractor (these particular belts actually need 4 anchor holes) and also two small ones (for the rivets) needed to be drilled per side on the top.

The most tricky part is to drill all holes on the top to match all the holes of the anchor plate and pull the anchor plate through the tunnel for the rear dome light, so I had to be creative and taped the plate to a measuring tape to pull the thing. 

The retractor actually is going to be hidden behind the rear arm rest, I'll need to cut it a bit or bend to make it look decent or will just use the ashtray frame as a tunnel, will figure out.

Everything is pretty much done, just waiting for the rivet tool to arrive so I could secure the top anchor plate like it should be.

Unfortunately the pillarless design is not much of pillarless, but safety is more important.

PS. I have covers for the top black segments of the belts, just didn't put it for a mock up assembly.