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I decided to go the 12v alternator conversion and am confused as to which size pulley should be on the alternator. Most kits include a 5/8'' pulley but when I measure the crank pulley I get 1/2'' measured between the pulley flanges at the widest point. I even put a 1/2'' belt on and it seemed to fit ok. So am I measuring wrong and do I actually need a 5/8'' pulley for the alternator?

You are not coming across very clear on what you mean by a pulley kit for the alternator. You will need to match pulley width so the belt will fit both. If you have the stock M38A1 double groove pulley on the crank then you'll need to find a matching pulley width with at least one groove for your alternator choice. The only other choice is convert to the early civilian F134 crank and water pump single groove wide pulleys.

What I meant by "kit" was the alternator conversion kit which includes the alternator, pulley, coil and mounting bracket. Yes I have a double groove pulley on the crank and I am just trying to confirm what I measured and that is that I would need a 1/2'' pulley for the alternator to match the crank. Thanks.

Just a heads up, even though the actual width at the top edges of the pulleys are 1/2" and 5/8" the applicable belt widths are actually much smaller at 7/16" and 1/2" respectively.

Also when sizing the correct pulley to use on the alternator one must also consider the diameter of the pulley which has a large effect on the come in speed and max output of the alternator.

So bottom line do I need a 1/2'' or 5/8'' pulley to match the crank? The kit did not specify what diameter pulley is included I just assumed since it is f134 specific that the correct diameter was determined. Thanks.

All three pulleys must have the same groove width!