Um, the simple answer is to pick a lighter cartridge with the same bolt face and have HCR change the barrel. Or have an extra bolt built for your Predator (about another $200-250) and you can run whatever long action cartridge you want.

A new Hart with 700 threading will cost you, what, $400-450 these days? So what if you end up with the extra .300WM barrel layin around, at least it would be ready any time that Elk hunt DOES come back in the picture. Besides, a rebarrel job on a 700 is something you could do yourself, so have HCR install both, then get the proper headspace gauges and you could swap back and forth in a saturday afternoon. For about $700, you could have an extra barrel and bolt to do a quick change over, with minimal money "wasted".

I have a Savage Precision Action in a Shehane Laminate that I use for hunting P-dogs, coyotes, and antelope. I have a .223rem, .243WSSM, and 7mm WSM boltheads and barrels all for the same action and stock. About an hour and I can swap between barrels/cartridges, a few shots on the range to confirm the zero, and I'm back in business... The Predator (a 700 drop in) is a little more time consuming, but still a pretty simple swap.