These old Model 70's in .375 are actually valued more as shooters than collectors, anyway. The .375 WBY is enjoying a kind of come back today. My guess is that, although it will not bring what it would if original, that being rechambered to .375 WBY will only knock 10% to 15% off of its market value as compared to having been left as a .375, most especially if the gun has seen some use and is in "shooter" condition and not in near-mint "collector" condition anyway.

Depending on condition and whether it is otherwise original (no chopped stock, refinish-reblue, or --god forbid-- muzzle brake), $1000 could be low.