Your alternator is out of the picture when starting the engine. Your problem isn't an alternator problem. There are a couple things you may have going on. First you shouldn't need 12.45 volts to turn over a 454, especially if it is a stock 72 which were lower compression, it should turn over easily with much less than 12 volts. I've got a fresh 12:1 small block right now that I start once and a while off an old car battery I have laying around and I know it isn't storing over 12 volts, it starts right up every time and I'm running the big flywheel and balancer so there is a little bit of mass to it. What you need to do is measure the voltage at the starter when you attempt a start and see how low the voltage drops. I suspect you have some resistance between the battery and the starter that is causing a significatn voltage drop. Before I do that I would monitor the voltage at the battery to see how low it drops when a start is attempted. No load voltage measurements don't really tell the condition of the plates in the battery. This is true for lead acid and nicads. We're dealing with lead acid batteries here and while they have a more linear discharge rate they do go bad and this may be your problem. See how low the voltage goes at measured at the battery under the load of starting the engine. Last if the voltage doesn't drop off too much and you are still having problems it may be a starter problem. Big blocks are notorious for taking out starters. Does this only happen after the car has been driven a while? If so you may need a heat sheild to protect the starter from the heat of the exhaust. Also check the alignment of the starter teeth with the flywheel. The only Chevy engines I have ever had to shim were some of my big blocks. They'd start fine cold, but once warmed up they were just tight enough that the solenoid would kick out, but not be able to freely spin. On the bench they worked fine, but installed they act this way until I shimmed them.