Total Timing?

Do you know how much mechanical advance is in the distributor? If so you can use a little math to figure out how much to shorten the slots, based on movement and degrees.

Total - initial = Mechanical. Get that number and when you bust into the distributor, you can shorten the slots yourself.

I've used JB weld to shorten the slots too. Easier to file than wire welded.

This is from a magazine article a while back.
----------------------------------------
dist. degrees X 2 + initial= total


dist. degrees / slot length
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
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Based on what you have I'd guess the slot length would need to be about .350"-.380"

Make sure you know where all the clips are located in the distributor before you tear into it. Get a copy of the factory service manual illustration if possible. I gave someone that "KNEW" what they were doing a 70 340 dual point distributor and it came back a bent useless mess, they missed the clip that hold the rotor standoff on the main shaft. They were prying on the thing and bent the plates.

It's easy to do.