Yup - buy the RUM, stick it in the safe, range out that old '06 for a while, then pull out the RUM when you're ready.

I know you don't want to "alter" your grandpa's rifle, but I'd venture that it has a wood stock. You COULD consider that bedding it into a different stock - TEMPORARILY - is a way to conserve the stock, with a happy consequence of a more long-range friendly stock. Guys do shoot long range every day with sporter stocks. If you could get comfortable with skim bedding and free floating that old rifle, you likely wouldn't notice much difference in accuracy between it and another stock when shooting from a pod or bags. Free float it, skim bed it (pillars if you're brave enough), and have the trigger tuned up and you might be surprised how well it'll shoot.

Brass for that '06 costs about half as much (or 1/4) as the RUM brass, and burns around half as much to 70% as much powder, so you'll be able to shoot around 3 for 2 on the same price.

Shooting 1MOA at 600yrds isn't mindlessly easy like it is to shoot 1MOA at 100yrds; it does take some development to get there consistently, but keep in mind - a 1MOA group at 600yrds is 6", that'll kill any deer or elk in the world.