Acording to the highlighted text, you wish to put a 9.3x62 chamber in a .35 Whelen. The title, however says you wish to re-bore, coupled with the text, into a 9.3x62, not just what is literally an IMP .35 Whelen.

The IMP Whelen is the way I would go.

Many years ago, Scovil and a fellow named Hawke put a 9.3x62 chamber into a .375 barrel, making what they called the .375 Hawke/Scovil. This chambering actually gave the ctg. a 78gr. caspacity, which is larger than the standard '06IMP chamber gives. The reason was the slightly longer body and the .475" base diameter. The .454" shoulder of the normal 9.3x62 was already the same as the '06IMP case. I know that some scematics say the 9.3x62's shoulder is .451"- but 2 I have measured leave fired brass at .454".

I may be going off on a 'different' tangent here, but after developing this ctg. Scovil had another reamer made up which had the same case body as the 9.3x62, but was made with the standard .470" base so '06 brass could be used. This "new" chamber was called the .375 Scovil. It differed from the standard (if you will) .IMIP case by having a longer body, shorter neck, but still had the sloped 9.3x62 shoulder angle.

What I would do, unless that .35 Whelen barrel is crap, if wanting more 'power', simply go with the Ackley IMP .35 Whelen, or the Brown Whelen,(same as Scovil's) which will give you a 77gr. to 79gr. case capacity. THAT is a good round. There are no flies on that one. If you wish, the premium bullets available today in .358", are super good and will do/perform however a 9.3x62 would.

As to a re-barrel, many years ago, this was very much cheaper than buying a new barrel and was highly recommended - today, as Steve noted, it can be just as expensive or more so than a new barrel. This was the case about 25 years ago when I was going to have a barrel re-bored and rifled. The price I was quoted was higher than a new barrel in the desired calibre, from the same gun smith - this was in Alberta somewhere.