Those old guns had different tube contours as the modern ones very often; they were more slender and diameter became smaller very short after the chamber cone.
Apart from the fact, this gun is proofed for B-P loads, there is a real risk that the thickness of the barrels will be very thin when the gun has been altered to the 2 3/4 chambering.
It is a very nice gun you have there, do not take any risk with it!