squarebridge,
I emailed Manson regarding the 350 G&H Magnum and also to confirm my 358CRG. Here's his response regarding the 350 G&H magnum, and its an interesting response.

"I've attached several prints of 35-cal Magnums for your reference, and a description of the 35 Magnums from the Speer #4 manual that covered wildcats popular at the time the manual was published. Reading the comments about 35-cal magnums, and comparing the different versions that have come down to us, it's apparent there was not just one chamber spec at the time. Chose the one you like and have at it!"

Manson's sent a few different reamer specs for what is a '350 G&H Magnum', which just goes to show that as wildcatters, we tread on the shoulders of great men that have gone before us. As gryphon notes, there's nothing a 350 G&H Magnum cant do that the 358Norma wont do ... and if you want serious horsepower ... the 358STA and 358RUM will do it in spades.
Cheers...
Con