Proofing is not a DIY proposition - ever!!! The value to firing a proof load is the preceeding and following inspections by highly skilled inspectors. Firing a high pressure load without the skill and tools to recognize damage (and some damage requires very exacting measurements to detect) may well set you up for a failure with a normal pressure load. Don't do it. "proof marks" and bore measurements are what you go by for determining state of in-proof. I'd not trust the proof marks, so, off the gunsmith you go.
An inspection by a known gunsmith should get you a go-nogo on firing. Note that 2 3/4" chambers are NOT an indication that the gun is suitable for typical SAAMI pressure loads. The gun should be shot with low pressure loads based on age and design.
Within the limits of whatchagot, this can be a nice, fun gun. Refinishing the stock and properly rebluing the barrels might be in order.
