Alright, lets clear up some stuff. My .300 wby can make a 1000 in its sleep, so there is no need to cram a bunch of any powder to get it to a 1000 yds. but you can if want. First I would like to state that the interlock is a hunting bullet and not a long range match type bullet. I would develop a load with one of the high BC match type bullets. Hornady Amax and BTHP Match, Berger VLD or Hybrid, Sierra MK. I would take advantage of the case capacity of the WBY. and push as high a bc bullet ( over 200 gr ) as I could. Yes I can send a 190gr MK at at over 3100fps but the ballistics are better on the 208gr amax @2950, plus I use less powder a little less pressure and longer barrel life. I found my rifle likes the 208 under 81.5gr of IMR7828ssc, but h1000, Retumbo, H4831, RL22, RL25 all could work well just depends what your rifle likes. I like the 208 gr amax because of quality and cost and high bc, you can't beat $31/100.
I don't see what freebore has to do with anything other than extra velocity. Yes some bullets don't like the jump but your Winchester may not have freebore. It sounds like your rifle is set except for two crucial items (maybe 3) first is your glass. You need a decent scope with at least 10x and enough elevation adjustment to dial up to 25moa. My load takes 23.7 moa at a 1000yds.
Next would be your trigger. You need a light crisp trigger to get consistent accuracy at those ranges. I think under 3 lbs. You maybe able to safely adjust yours down that far but I'm not that familiar with the Model 70 trigger. Mine is set right around 2.5 lbs, I can go lower but this is still my hunting rig I don't want a trigger any lighter for hunting. A quality bipod would be the third thing but you could always shoot of bags without one.
Once you shoot a 1000 yds with this you might get the bug to take it farther. You should be able to take it past 1800 ballistically. You could always do more but then it wouldn't necessarily be a practical hunting rig anymore. Good luck.
