John, I've always used belts from Kanter in my Hudsons. I note that they have them in tan color. https://www.kanter.com/

There are several aftermarket seat belt companies so you might look around to find the best color match for your upholstery.

When bolting the belt anchors, be sure you locate them correctly beneath the rear seat so that the "pull" is in a straight line from the passenger's waist, down to the portion of the floor to which the belt is bolted. If the anchor point should not, for example, be located toward the front of the seat. In that circumstance the belt would loop around the back of the seat, then come forward to the anchor point. In an accident, the passenger's body will actually pull the seat forward several inches before the belt becomes taught, with catastrophic results.

Also: consider installing a retractor to pull the belt latches back to the "crack" in the back of the seat cushion when not in use. This will keep the belts neat and less noticeable, and they won't end up hanging out the rear doors. These can come in the traditional form:
[img ] [/img]

or in the form of an inertia retractor at the anchor point, under the seat:
[img ] [/img]
My guess is, however, that this inertia retractor will be too large to fit under the seat.

Lastly: when bolting the anchor to the car's floor, buy or fabricate a large, thick steel "washer" for the bolt, which will reside beneath the floor plan. That's very thin metal to which you're bolting, and it could be compromised by rust. You need to spread out the forces generated by a 150-pound body going forward at 40 mph, which is going to want to rip that bolt right out of the floor. The shape of the "washer" will depend upon the location of various obstructions near the bolting point (steel braces, parts of the frame, electric or hydraulic lines, etc. It may take a while to engineer this item (as opposed to simply drilling and installing your bolt) but always keep in mind the lives of the passengers in your Hudson.