I also added the three point system to my 54 AD and if you've ever seen the results of a frontal collision with an AD solid steering column and a driver you would too!! Not that a 3 point is going to keep you off the steering column, but at least you're going to have a fighting chance of staying with the vehicle in a less severe collision.
A couple things I noted. I used Deve's technique for my installation but found that you need to move the upper mount hole back if you want to be able to run your door windlace in the provided (54) track. On earlier models you may have to cut your windlace. Secondly, when I ordered my belt set from Juliano's, they didn't send me the extended length belt for the buckle end. Had to add a piece of flat stock from the floor up to get the buckle onto the seat. Ask them for the extended length belt.
Aside from the safety issues I've noted that many states are allowing law enforcement folks to pull you over if they see you are not wearing your belt. With only the lap belt there is no visual clue that you're belted in, and I personally don't like to open the door to some overzealous officer who might want to pull me over.
The time to install belts is early in your build, not as an afterthought. I mounted my floor anchors behind the seat pillar. With the tank out you can get a much cleaner install. I hope this helps.
Dave
