I ran into a similar issue last year so I may be of some help with your question-
To preface- I was shooting an M1 Garand and did fine for the first 3-4 clips but in my excitement of shooting it, for just one time, I didn't have a proper shoulder mount and that steel butt plate just caught my clavicle in the worst possible way and ended my fun for that range session and left me with a gnarly bruise for a couple of days last May.
The slip-on recoil pads that have been previously mentioned absolutely work and if you're ok with that option then nothing further needs to be said. There's also 'grind to fit' options from limbsaver at least (I'm sure that there's other manufacturers that offer similar products). That said, I can only speak for myself and not trying to infer your own personal thoughts on the matter, but in my experience the slip-on's added too much to the LOP and it always bothered me to throw that on a classic rifle myself. The 'grind to fit' pads were a little bit better but again (for me) just was difficult to do that to a classic rifle as it just seemed to wreck everything that rifle should be in its original configuration. You might not feel the same way but just sharing my internal struggles with those two options is all.
What I settled on after some experimentation was a PAST recoil pad that you place on your shoulder (I'm a righty but apparently they're ambidextrous). That took the 'bite' out of the steel butt plate for that style of rifles while keeping them true to their original configuration and solved the LOP issue as well. The PAST shoulder pads, last I looked at least, had various thickness options, but that allowed me to continue making expensive noise from the old military warhorses while only wearing a t-shirt in the summer (didn't seem to be a problem for me in the fall/winter at least when I was wearing more than a t-shirt).
I hope that's of some value to you- the last thing I'll add is if you'd decide to experiment with them- they're rather inexpensive, something like $20-30 and it wouldn't screw up any restrictions on the rifle itself (assuming on my part that you're trying to follow CMP vintage sniper rifle rules).
-LD
