Spend money on seatbelts that are both
new and
good, then hope you never have to get your money's worth out of them.
I used to recommend retrofitting 4-point Wesco belts as I did on my '71, but
not any more (the article is correct in its facts and physics, and everything said about 5-point harnesses is equally true of the 4-point variety, except the additional injuries that a 5-point can do to your 'nads).
When I get rid of the separate lap/shoulder straps in my '73, I'm putting in
these up-to-date, reputable-brand ECE-approved 3-point seat belts (linked vendor is reliable; I've been buying there for a decade). More info on mounting at the
maker's website. If I needed mounting hardware, I'd buy it again
from Wesco (and their page has good illustrated write-ups on how to install belts in pillarless cars like yours
here ), but the Securon belts are a much better-designed product than anything Wesco carries.
There are also bench- and bucket-seat 3-point belt setups available from
XV Motorsports, but I am hesitant to recommend or use them until I have more than a one-line say-so from the company that their belts actually meet the relevant safety standards; discussion on that matter is
here.