It really depends on your end goal.
I have a "kinda sorta custom" M70 in 35 Whelen.
M70's were never made in 35 Whelen. They WERE made in 30-06, though, and a place called JES Reboring will ream barrels and re-rifle them to bigger calibers/chamberings. I had my 06 reamed to 35W. I took the nasty plastic stock off and put it in a McMillan and pillared/bedded it. Cost of the rifle, plust $300 for rebore and $350 for the Mickey (got it second hand).
One of my very favorite PA deer rifles is the result....
If I were building one from scratch, since I'm a big M70 fan, I'd look for a pre64 action, a quality barrel, and have a smith with a known and good track record assemble/lap/true everything, then pillar and bed it into either a stunner of a walnut stock or a McMillan, Manners, or the like.
And while he's got it, I'm gonna have him mount/lap/true the scope rings/mounts and get that all squared away. Not that I can't do it, but if he's got it on the lift, so to speak...
As for chambering, I'm not sure what I'd personally do. Probably either a 338WM or a 375H&H or WBY. I don't have anything "above" the 35 Whelen, but I've got a ton under it. So that's probably what I'd do.
From talking with folks who have built rifles from the ground up, so to speak, they generally wind up at 3500-5000 for the final, finished product.