The actual lifter spacing on a FE is 1.98 and the valve spacing on factory, LR type heads is also 1.98. I asked about this on this forum recently and didn't get a answer so, I had to research it, before finding the spacing. I verified it with my engine. So, if your rockers are 1" wide to be centered on factory valve stems, you need a distance of 2.98, from outside to outside of the pair of rockers.
Because Edel is using a .060 spacer, on the intakes, I would assume, that is the amount they moved the intake valve. The rockers don't have to be perfectly centered but, I wouldn't deviate more than +/- .015 for each rocker.
For reference, I believe the TP, HR & MR factory valve spacing is 2.10. I don't have one of the blocks to check but, I think the lifter spacing of the large valve blocks is the same as the LR style at 1.98 but, I can't verify that.
If anyone has one of the above blocks, on hand a caliper measurement of a pair of lifter bores, outside to outside, would verify that. If it's 2.885, it's 1.98 C/C. If it's anything else, subtract .875 from your reading and that will be the C/C distance.