ok, the cam mfg's don't know the base circle size but that's ok.
Did you rotate/unthread the nuts on the screws at all or did you just loosen the nut then turn the screws out?
How far are the rocker shafts from being bolted to the head, 2 mm 4 mm etc?
From the looks of it so far I think you should buy a cheap 6.80" and 6.75" long push rod and try both of those.
1. Remove rocker set from number 1 cylinder and clean the top of the valve stem with cleaner.
2. Color the top of the valve stem with felt pen or dykem.
3. Install one push rod on the intake valve and the other one on the exhaust valve then tighten rocker shaft bolts.
4. Adjust to .021" and tighten nuts then take photo of adjusting screws.
5. Rotate engine clockwise 4 full turns.
6. Remove rocker set and measure the width of the pattern on both valve tips.
7. Take a close up photo in which we can see both valve tips at the same time.
If the pattern is definitely narrower on one tip, you want that push rod length that was on that rocker.
If the pattern is similar, you want the push rod that makes the pattern closest to the center of the valve.
This will get you plenty close enough for your app.