I have never measured the inductance of a power transformer in my life, and I been in electronics since 1954. Not saying you are wrong, but I wouldn't know what to do with that data. The transformer connects to 120vAC and produces reasonable secondary voltages? And it doesn't draw excess current? Sounds good to me.
The Gibson expects 260vAC on the HT winding. Your have 300v. So your B+ will be higher than stock. 420vDC rather than 360vDC. More or less. Probably not a deal breaker. I don't stock chokes, I could probably find something well suited, but at least for a working project I'd stick a 400 ohm 20 watt, or something like that , in the spot. it is just a guitar amp, so you have a wide range of acceptable values.
Northern VA? I grew up across the Potomac from you. Spent a year in Doswell for that matter.