Is it not the angle of the rod/rocket arm changes when using a different cam? I mean look at the tappet adjusters, Then causing them to run closer to the walls
Or
Cylinder pressure vs 1980 push rod metal
I looked into push rods from America but was going to correct the length but you need a adjustable length test rod then adjust and check the top of valve for centering the rocker arm on the valve etc
Everyone's head is different height different gasket so you can't really bulk buy
If I were doing a big power C1J, I would use a billet cam, not a recut, to reduce the pushrod length problems.
Look at bespoke pushrods, as they seem a weak point. Certainly open up a channel in the head if you can to accommodate a thicker rod, make the rods from titanium or Nimonic for the best strength/weight ratio, i'd also look at the back edge of the rod to see if I could add material behind it NOTE: All of this might be bollocks, but i'd try anyway.
I'd also try and combine that with bespoke tappet adjusters and give myself lots of adjustibility on length, and look to join the pushrod top to the adjuster to eliminate the chance of the adjuster unseating if the valve floats at high boost.
Depending on the variance of length of adjuster, I might make the pushrod adjustable instead, with a fixed tappet (non) adjuster. I might even look at bespoke rockers so I could do stuff like extend the tappet adjuster back a bit to help with pushrod clearance in the head, which I could do because I could compensate for the slight rocker ratio change with my bespoke billet cam... 5mm of lift on the cam x 1.6 ratio = 8mm valve lift. 6mm lift on the cam * 1.5 ratio = 9mm valve lift (blah blah, you can fuck about with numbers like that for days).
...and so on and so on. Its all do-able if you want to, and if you have the time and money to invest. My cylinder head took four years to spec and build.