Slots: Slots are often at a slight angle. Thats OK.
Length: 15x 2 = 30 degrees of advance (distrinbutor turns at half the crank speed, I wrote it down backwords above)
Weights clearance to body: Yup - its small. Just make sure there is no wobble that would let them hit!

OK: It sounds like you found the problem. If I understand correctly..
Are both of the springs are loose when the weights are pushed in? That is, when the pins rest against the begining of the slots, is either spring helping hold it in?

Even if the the light spring is providing a little bit of pressure, what may be happening is that by idle rpm, lets say 500 for simplicity, the mechanism has already advanced 25 degrees. Now there's only 5 degrees left in the slots. Setting the initial timing of 18 at idle, the distributor advance then tops out at 23 degrees. It probably hits 23 by 1500 rpm.

Take the other distributor apart and see if you can figure out why the springs in #1 dizzy are not coming into action as they should. Wrong springs for this application, or someone bent the loops, or something.

When set up correctly, the light spring should be strong enough to keep the weights from moving out unitl 700 - 800 rpm or even a little higher.

Exception is the old Direct Connection setup where they purposely used a super light spring with that long looped super heavy spring. Made it near impossible to set timing at idle. Timing was set at 'full' and everything at lower speed was just accepted for whatever it was.

You could use a similar approach to get around shortening the slots. But whether you do that or braze up/weld the slots, you first need to get both springs to help restrain the weights.

When you think you have it figureed out, install and measure the timing at 100 or 250 rpm intervals all the way until it stops advancing. If its going about 2500 or so, feel free to make it 500 or 1000 rpm intervals. be careful!