You're starting off the hard way.
I'm assuming you have everything disassembled, so to measure the required stack height:
1. Measure the length of your steerer to the bottom of the crown race seat.
2. Measure the length of the headtube on your frame.
3. Subtract #2 from #1. The result is the MAXIMUM stack height headset you want. Anything shorter you can make up with spacers.
4. Measure your fork crown [as you indicated in the post above]. If it's 26.4 then you have an ISO headset. Skip step 5. If it's 27.0, you have a JIS crown - but it's possible you have either a JIS or ISO headtube. You can buy mixed headsets - I've had to buy 'em. Proceed to step 5.
5. Measure the ID of your headtube [where the lower cup/upper race seat] or the insertable OD of your headset cup/race to determine the proper headset standard.

You will need a digital caliper to make these measurements. Harbor Freight currently has their 6" model on sale for $9.99. I own one - it's perfectly fine for a bike mechanic. I probably wouldn't use it on my Biturbo crankshaft.