Rich,again,in 68 GM DID NOT HAVE HARDENED EXHAUST VALVE SEATS unless they were installed in prior rbld you spoke of.
But even if you dont have hardened seats i ran my 396 for 25+ yrs without hardened seats but did run the inexpensive valve/lead additive and the exhuast valves did not receede at all into the head when i tore it down for rbld in 2002.
Its when the motor is under heavy loads like towing or sustained high speed crusing without the hardened seats that makes the ex valve seats to start wearing away/receede into the head.
Since your in doubt just run the inexspensive lead substitute and dont worry about it. I used to get a bottle of lead additive that treated 320 gals for approx $10.00,cheap insurance.