I removed the terminal block on my starter and I have plenty of room to get at it on my 63 as you can see it has alot of room on the drivers side. I can go right between the header tubes and get on the starter terminals. But I am lucky to have the room as you may not as you have stated. The 11 volts while cranking is good but you said it dropped more then that first and then came back to 11 or so volts. I would want to see the starter draw when it first drops and when its cranking showing 11 volts. I have seen starters with worn bushings do that where on the hit of the key they drag at first and then once they turn some they will center some in the bushings and crank better. From what you said it sounds like it could be in the starter but you can tell more if you see the starter amp draw at first hit and while its cranking. If it draws way to many amps at the hit it I would suspect starter dragging causing to much draw. And if any way to get to the starter terminal where you put the voltmeter positive on the battery positive post and the voltmeter neg on the starter hot 12 volt feed terminal and should have no more then .5 volt drop when cranking. I have seen them as high as 7 volts with a bad cable or connection. Is there any way you can put a 1 to 2 ft jumper wire on the starter hot stud when the eng is cold and leave it on there and put the other end of the jumper off to the side but insulated so it dont short to ground so that when its hot you could just hook your voltmeter to the jumper wire so you dont have to get around the hot header ?? Then do a voltage drop test also when hot. The voltage drop test and the starter draw test will tell you a ton about the starter circuit. Ron