dist. mechanical advance ajustment?

Welding the advance slots and filing or grinding to appropriate length seems to be the most common method. Here is a chart for slot length that should get you close.
Dist deg. Crank Deg. slot length
6 deg 12 0.340
7 deg 14 0.357
8 deg 16 0.375
9 deg 18 0.390
10 deg 20 0.405
11 deg 22 0.420
12 deg 24 0.435
13 deg 26 0.447
14 deg 28 0.460
15 deg 30 0.475
16 deg 32 0.490
17 deg 34 0.505
18 deg 36 0.520
There are also these plates that some folks use.
How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races
Mopar Distributor/Mechanical Advance Timing Limiter | eBay