You're killing me.
First of all, you're going to end up with a 300 win mag or 308 Norma. But more expensive brass and dies.
If you're going to deal with forming brass and using custom dies, you're going to be kicking yourself once you do want more speed. The cost of adding a reamer onto this is probably not as much as you think. You could have your own cut, probably for around $125 US - at least on this side of the pond.
That's about the cost of your dies, which will be paperweights once you rechamber to Improved.
30-8x68 Imp would be one mean magnum, but a straight neck-down won't give you anything you can't have cheaper. The 30-8x68 has about 85g of water on a good day. Improve it, even with a decent taper left and you'll get more like 95 or 97. See what I mean?