Have a chat with diamondhell about harnesses though. Apparently they are not as safe as a seatbelt unless you have them strapped onto a harness bar.
While fitting in some random Subaru seats, using the original seat mounts isn't a terrible idea, it will need to be done carefully, to ensure what you're doing is strong enough that it won't collapse in an impact. How confident are you in your welding? It has to be properly done, not bird sh*t-stylee rubbish.
Unless you are competent and getting sufficient heat into your welds and penetration making up your own seat-mounts isn't something to be undertaken lightly...... it would be a shame to see some sort of Darwin-esque incident involving your Golf and the deletion of a small number of IQ points from the gene pool.
As for the harnesses, really you need 5/6 point, including a crutch strap and for them to be properly effective they need to be tight enough that you can't move about comfortably in normal road driving. If you can then I'll wager the set up you have won't retain you any better than a factory 3-point belt.
If you're going to replace your 3-point belts you need a minimum of 3" shoulder straps. If you go for 4-point harnesses to go with those seats (because there's no crutch strap hole, because they're designed for use with a 3-point belt) then be aware you're at risk of submarining down under the harness, doing a lot of damage to your back especially if it's not properly tightened, because you're 'just driving about on the road'.
You've also got to ask yourself 'what will happen with the shoulder straps in an accident?' because those seats are designed to be used with a 3-point belt, not a harness. The shoulder straps should really be as short as possible and secured within an angle of 0-10 degrees downwards from your shoulders. Is the hole in the seat even designed to accept a harness? I doubt it very much, because - here we are again - they're not designed for harness use.
So, you have a single threaded hole on the outside of the car, which is designed to accept the waist strap on the outer side, but the inner seatbelt mount (the clip) is mounted on the seat frame on the original set up, so when you slide the seat forwards and backwards the whole fixed harness will twist across your body and need substantial adjustment to ensure it's correct whenever the seat is moved.
You'll need to weld a spreader plate on the underside of the tunnel (not the inside) to secure the inner waist strap mounts.
Now what of the shoulder straps? You've got nothing to use in the rear, have you? The top mounts for the rear seats are miles away and off-centre, so you'll be dragged across in an impact. The bottom mounts are way outside the maximum 45 degree angle downwards, plus if you use them, on an impact, as the seats aren't designed to take a downwards force on impact the back rest, the harnesses are going to pull down on your spine and crush it - yay back problems for you. Your only option is to fit a 'proper' harness bar - tubing of the correct diameter across the rear turrets, with appropriate spreader plates to stop it tearing out on an impact.
Even suggesting you could use a strut brace to mount your straps to is utterly laughable. It's designed to stiffen the car, not take any form of impact. You'd be a complete moron to ever contemplate using one as a mount.
So, possibly a little more work than you're expecting and something that is not going to be comfortable, or sensible in a day to day car.
You'd be far better off retaining the standard 3-point belt and getting
one of these for enthusiastic driving.
