Any light source is subject to falloff off-axis.
I raise the enlarger head (diffusion) way up for a giant print size. The little 4x5 negative is in the middle of a comparatively huge imge circle and within that 4x5 area I don't have variations of even 1/10 stop. That's good enough for me. But if someone prefers to use a softbox 30 feet from film taped to a wall that's probably good too.
Anyhow best of luck to OP. I would be curious to hear about your results with the Mod54.