I don't know who made the barrel, but I SERIOUSLY doubt that it is Remington. Remington makes the .35 Whelen in their custom gun shop, but the barrel is not stainless. As such, the barrel that you saw could have been made from just about anyone. Best bet is to look closely at the barrel. I bet you that the name of the company must be printed on there.
As far as making it an ackley improved model, my understanding is that you would essentially have to get it rebarreled again, which should cost the same for an ackley improved version as any other cartridge.
Zachary