If you cut at the neck were would you seat the bullet?? another alternitive for a large bore in that length would be necking up a 358 win (358win is a necked up 308) to .375.
I have some RCBS 44 mag dies coming to play with on the 44???. I think I will try to fire form the cut 06 brass in my 444 marlin dies to blow out the case then try to seat and crimp with the 44 marlin dies. Once A actual gun it built then fire forming can be done in it. I think the 06 brass can be taper expanded as well. I have been wanting to build one of those in die fire forming set ups anyway. It might be possable to just cut the 06 brass to size expand the neck the neck, seat the bullet with 44 dies then just shoot it in the built gun plinking with lead bullets. the fired brass will then be blown out. the catch 22 is you need a round that is all formed and finalized to design the chamber off of.
I am thinking a throating reamer can be used first then follwed by boring the sligtly tapered chamber in the lathe by using the tool post set at the proper angle. how ever the chamber is cut, it is going to need a very prounced step from chamber to free bore to headspace off of.