What, EXACTLY, is the chamber length of your 20g gun.
British 2 1/2" chambers are (usually) just that; 63.5 mm
European 65 mm chambers are 2.56"
European 67 mm chambers are 2.64"
U.S. 16g 2 9/16" chambers are 65.1 mm
12g 2 5/8" chambers are 66.675 mm
12g 2 3/4" chambers are 69.85 mm
Some just after the turn-of-the-century U.S. maker 20g chambers were cut slightly shorter than 2 1/2".
.....2 3/8" = 2.375 = 60mm; 2.4" = 61 mm; 2.45” = 62.23 mm.
Short non-boring answer: Sure, if the 65mm load is "safe" in your gun, the 67mm will be.
This table deserves a sticky of its own.
--
Moving on to the sharp "step" forcing cones 2-piper mentioned, I ran into those in a pre-revolution 20 ga Tula hammergun I examined a few years back. There was no "cone" there, just a step from chamber to bore at 2 1/2 inches. Of course, the prior owner had been running 2 3/4 WallyWorld promo loads through it since time out of mind, but being a Russian gun and stoutly built it was none the worse for the wear. Internally at least. External cosmetics were, shall we say, a bit rough.
