If your original seat is in decent shape it may be better to just have it recovered and then you know it will fit without any modifications. That is what I did.
As for seat belts: I installed 3 point versions (with a center lap belt) from Seat Belt Planet.
https://www.seatbeltplanet.com/You can use the existing attach points on the floor for most of belts attach points. Due to my repairs of the floor plan I had to drill the outside attach holes for the 3 point harnesses (2 each side).
If you purchase the hardware from Seat Belt Planet you will get everything you need to make the install.
I will have to take some pics and post them later.