Distributor advance

Here's a chart to give you approximate slot length vs advance degrees


Bear in mind this is DISTRIBUTOR degrees which is half of crank degrees. You original "top hat" is probably stamped with the original mechanical advance figure. Again it will be dist. degrees. So for say, 20--22 at the crank you want a slot length (below) of .4-.420

Dimensions for Mopar distributor slots, in DISTRIBUTOR degrees

Modifying advance slots
degrees / slot size
6.............. .340
7................355
8................375
9................390
10...............405
11.5 ...........420
12...............435
13...............445
14...............460
15...............475
16...............490
17...............505
18...............520