I suppose part of the limitation is the amount of material around and between the various elements of the construction, with little need for the original designer to plan in space for adaption.
Interestingly, most things like this appear to be clouded by "round" thinking as most solutions head towards something like plain bearing shells to offer a solution.
If there's enough meat in the supporting casting you could consider milling out the originals to square form, then making phosphor bronze blocks that wouldn't rotate anyway. If you made the bottom one wider (length along cam journals) than the top, then you'd get a staggered joint that would be pinched by normal assembly. Pairing the half blocks and line boring them would give you cam clearance dimension. Close, not interference, would produce head casting fit.