Scott,
let me help you with this a little bit. first, use the 'T' bolts that are made for the belt trim ... they fit into the groove on the back side of the belt trim and slide so you can align them with the holes in the body. there are 47 if i remember correctly. The T bolts have thin heads that are narrowed to fit into the groove
second, if you have recently painted the car, before you have the belt trim installed, wait as long as you can up to a month. too much pressure on the paint will cause it to ripple around the belt trim which will damage your fresh paint.
third, get the belt trim shims from Carl Nelson - these shims hold the belt trim off of the paint slightly. you put them on the 'T' bolt just before you install it on the car.
fourth, use new rubber backed nuts to install the belt trim - they mount within the body cavity on the other side of the sheet metal. the rubber fits between the body and the nut and will seal the holes to keep water from infiltrating the interior thru the holes that the 'T' bolts go thru. when tightening the rubber backed nuts, tighten them to hand tight, not much more ... this is what can cause your paint to ripple. you don't want them too loose or they can vibrate off ... you want to flatten the rubber slightly.
the T bolt is part number -
51135780108 + getbmwparts lists them for 3.30 each. the nuts listed on realoem are [URL='http://www.realoem.com/bmw/enUS/part?id=3435-USA-03-1972-E9-BMW-30CS&mg=51&sg=20&diagId=51_3429&q=51135780115']51135780115, and they are listed on getbmwparts for .65 each, which seems a bit inexpensive for a rubber backed nut. that is also the parts numbers listed in the orange books.[/URL]
you can buy a set of everything you need from W+N for 122 euro
https://www.wallothnesch.com/en/set-of-trim-line-fasteners-with-nuts.html