Main caps are machined to each engine. They must be installed exactly in order (#1,#2,#3 etc.) and are left or right. To install non-original caps, considerable machine work is needed to mate those surfaces together again.
No bud. On an early hemi 1-4 are identical. Yes, they have numbers cast into them indicating which main they're supposed to be on, but they work exactly the same regardless of their position. "considerable machine work" is not necessary. All that is needed is a line bore, which is not a completely unusual thing to have done to an old engine even with its original caps if it's being built for big power. If a significant line bore is needed you may need additional work so that a shorter timing chain isn't needed, but even that isn't what I would consider "considerable machine work". All of it is fairly straightforward, and is not uncommon for guys that build the early hemi's.
***Edit***
I want to make one change, I went out to look at my collection of Chrysler hemi mains to confirm. The #3 cap is chamfered because the bearing has a lip on both sides. So a #3 cap could be used in 1-4. But a 1,2, or 4 cap would need the chamfer added for it to be a #3 with the correct main bearing. The 1,2, and 4 main caps are identical.