Before giving too much guidance, are you trying to install the upper pulley on the V8?

OK...first take some Emery Cloth lightly to the shaft to ensure it is perfectly smooth. Removing the old pulley might have left some very minor scoring. Regarding the oven, crank that bad boy up to max, maybe even broiler setting with the door closed. If the aluminum pulley is sufficiently heated to slide on, the steel shaft will easily absorb the heat without the temperature rising too much for the seal to handle. The specific heat of steel is twice that of aluminum, so that shaft will not even get close to a temperature that will endanger the rubber seal. Use some form of spacer as a jig to properly position the pulley. You’ll get only one shot at sliding it to the right spot. Silicone oven mitts will be your friend.