I added a brake to my Coker much the same way you are describing this link
Yes, you do want to mount the brake on the rear, that way the mounting bolt only holds it on and does not get stressed by the braking action.
With the difference between the amount of braking you get and the amount you need, I’d say just let nature take it’s course, I use one finger very, very lightly and get plenty of braking. Just make sure that the shoe is parallel to the rim in the direction of movement.
One other thing: with a Coker steel rim you will probably find it to be a little disconcerting when the weld comes around, that’s one of the reasons I like my Airfoil rim. I was thinking (depending on what the weld is shaped like) that it might be possible to smooth the weld with an epoxy, it wouldn’t take much and I don’t believe the stress would be that much as the amount of actual braking isn’t as hard as trying to stop a run-away mountain b*ke.