I have a gunbuilder who is building a 375 Ruger on a pre-64 M-70. We are considering machining stripper clip grooves into the action if we can find or modify stripper clips.
Any ideas?

There is a lot more to it than that. Take a look at how a model 98 operates. The hump is needed support the stripper clip while you push down on the cartridges. Little tabs on the sides of the clip fit into detents on the sides of the slot, keeping the clip from going down to far. When you close the bolt, the clip is automatically ejected from the action.

The clip itself must be made of spring steel that gives, but needs to be strong enough to support the cartridges in a column. That gets difficult to do once you get above standard size cases. Heavy bullets make it even more difficult.

These might work:

I am well aware of what is required with stripper clips and use them with my 30-06 and 9.3x62. I am also working with one of the nation's premier builders who is also well aware of what it takes to make a rifle work.
I am just hoping that someone out there has knowledge of stripper clips.

Try Enfield clips, .303 Brit has nearly the same rim diameter, though the thickness might be different.

do you have any 303 british stripper clips? the rim is 540 and clips easy to come by.

I doubt your going to find anything that will match the size you need. Your best bet is to cut some 8mm mauser or 30-06 strippers( assuming the rim thickness of the .375 Ruger is close to that of a 8mm or 30-06) in 1/3 width sections lengthwise and Tig weld in a new center section of sheet steel to give you the right width, then grind smooth. Probably have to make a new spring but thats the easy part. Its actually easier than it sounds and you only need to make a few anyway. I've welded aluminum beer cans together and there are way better TIG welders than me around. Some so good your stippers could look like factory made.-Rob