A 7448 should work fine (or better) anywhere that a 7447 can, but I don't know where to get them. New 7447 chips are for sale on ebay.
Amazingly, documents like this are available, even today:
www.nxp.com/docs/en/application-note/AN1812.pdf
In there you can see that pin-compatible chips are not necessarily interchangeable. The Pismo is the only G3 machine I have for testing, and it seems to be a transitional example because it has correct configuration connections in place to easily take a G3 or G4 in 60x or MPX bus mode. I would guess that earlier G3's potentially have more problems. I think a 7447 working in a G3 machine that can be tested in either bus mode would be a step towards upgrading earlier versions that did not already anticipate G4 chips.
Your idea about putting a G3 into a Gigabit Ethernet might be another way to test things. I suspect it would work and firmware patchers exist for these machines. So testing a 7447 in 60x mode installed on 7400 pads in a Gigabit Ethernet could show the feasibility of doing something similar for earlier machines.
Amazingly, documents like this are available, even today:
www.nxp.com/docs/en/application-note/AN1812.pdf
In there you can see that pin-compatible chips are not necessarily interchangeable. The Pismo is the only G3 machine I have for testing, and it seems to be a transitional example because it has correct configuration connections in place to easily take a G3 or G4 in 60x or MPX bus mode. I would guess that earlier G3's potentially have more problems. I think a 7447 working in a G3 machine that can be tested in either bus mode would be a step towards upgrading earlier versions that did not already anticipate G4 chips.
Your idea about putting a G3 into a Gigabit Ethernet might be another way to test things. I suspect it would work and firmware patchers exist for these machines. So testing a 7447 in 60x mode installed on 7400 pads in a Gigabit Ethernet could show the feasibility of doing something similar for earlier machines.