I've been a Winchester collector (primarily lever actions, though I've dabbled in the pre-'64 M70 too) for enough years to have some opinions.

Ignoring the collector aspect for a minute, it just doesn't make much financial sense to cut the barrel on a pre-'64 .375 H&H unless it has already been modified. If it's in original but used condition, you'll end up spending money to reduce the total value...just doesn't make sense to me. It has already been mentioned, but you'd come out far ahead if you sold the pre-64 (without rebluing it first...that will also reduce the value) and bought one of the newer controlled-feed M70's. You'd also have better metallurgy and an action that's really long enough for the .375 H&H round.

I'd think that there would be enough collector demand in Australia to make that work out. Also, there's a definite "floor" on pre-64 M70's because people like to use those actions to make custom rifles. Bobbed barrels & cut/replaced stocks don't affect that price.

If this is a sentimental piece that you want to modify then by all means go for it. At least you've asked the question and know how it's going to affect the value.