That is CRAZY talk! You could blow yourself up, and maybe a bystander or two.
I would never ever substitute 308 Win or 7.62x51 for 7.62x54r, regardless of case dimensions. It is incredibly irresponsible to even suggest it.
The Proof load for Mosin Nagants was 3600 atmospheres, about 53,000 PSI. Military FMJ loads were about 43,000 psi. (Finnish M39 data)
The proof load for the 7.62 NATO is rated at 67,500 +- 2500 PSI average
The Average Pressure for M80 ball is 50,000 PSI. This approaches MN PROOF load data.
The SAAMI Maximum Average pressure for the .308 Winchester is 62,000 PSI. This EXCEEDS MN PROOF LOADS!
.308 Win Min Proof 83,000 PSI, Maximum Proof 89,000 PSI.
.308 win Data from ANSI/SAAMI document Z299.4-1992 Pressure and Velocity, Centerfire Sporting ammunition