Should just put those seats in it, and then the belts would work great. I considered that design at one point, and you could just modify the shoulder...
I can see how the non-sliding buckle could be a benefit over the sliding buckle and might even like it better after you get used to it. The lap belt...
As long as the subject is about seatbelts, which people might be looking at the thread for ideas, what's wrong with tossing an alternative idea into...
IDK I guess, but you mentioned 4 vehicles that you owned that you liked the factory seatbelts in. I tried to make mine better for holding me strapped...