"...only 50,500 CUP..."?????
That's not a trivial amount of chamber pressure! Most modern cartridges that are loaded to "only" ~50,000CUP (ie .270, .30-06, .308, etc.) are close to 60,000PSI, or higher, when measured with the piezo-electric transducer. Don't let the difference in the measuring systems confuse the issue.
If your .358 Win loads (or anyone else's for that matter) are exceeding full .350 Rem Mag velocities, then they are WELL in excess of the SAAMI pressure limit - I guarantee it.
The only way to get much more out of a cartridge is with the loading methods used by Federal and Hornady for their 'High Energy' and 'Light / Heavy Magnum' loads, respectively, where they are compressing far more (slow) powder in the case than the casual handloader can.
Note that the .350 Rem has a pressure limit of 53,000CUP, one of the highest limits in the industery - and just a bit more than the 52,000CUP of the .358 Win.
Yes, a very long barrel will increase velocities somewhat. But burning only 50 grains of medium speed powder in a .358" bore is not going to yield any fantastic gains beyond 22 or 24 inches.
I don't know who Mr. Conley is but I would be skeptical of any claims that it is possible to substantially exceed the published loading data, without unsafe pressure.
Mr. Conley's claims just don't pan out.