Are you asking because it won't start?
Find TDC by CAREFULLY sticking a wooden dowel into the spark plug hole and rotating the motor. Because of the angle the spark plug sits in relation to the cylinder, you can't really get any great accuracy doing it this way; but because you're only looking to make sure where the number 1 plug wire goes, it'll get you close enough. Then which ever spot your rotor is pointing, put your number 1 plug wire there. I've never done much with type IVs, so I'd assume there is a TDC mark on the fan, but am not sure where you'd need to look. Then just put the remaining plug wires where they need to go in relation to number 1 on the cap. Then start it up and reset the timing.
Also, be sure to check out ratwell's site. It's got a TON of great information about bay windows. http://www.ratwell.com/