Probably useful for internal inconsistencies which might require removing wood or of perhaps shimming. Externally visible differences going to aesthetics more your call. 'Most' all standard length mauser actions have the potential of substitution with just the cautionary matters mentioned. "Aftermarket" alterations or accessories installed such as trigger assemblies may be yet another matter in terms of complications. Bottom metal differences often with implications of aesthetic variations as also barrel channels. In other words, sort of hit & miss, most all of which can be structurally cured often with bedding materials. The devil in appearance.
"FN" mauser actions range the spectrum of "intermediate" size such as most often encountered Romanian mausers & their Model 48 Yugo successors, to the couple clicks lengthier standard size postwar Commercial actions with stock implications of solid left sidewalls (thumb cut in military stocks as possible tilt. To low scope bolt handles which may not comport to the stock wood handle configuration. The barrel channel is likely the greatest wildcard! I'd simply expect open the stock channel as needed. Or use "glass" filler or combo of both to reduce size! If properly accomplished "glassing" can appear quite innocuous unless filling Grand Canyon gaps!
Above off the top of my head, conjuring.
Good Luck!
Best!
John
