Making power out of the 318

you're not wrong, you're just floating around in the non-sequitur lagoon matey.

the correction that the speedo gear makes is about the furthest thing this dude should even be thinking about at this point.

it has no bearing, effect, matter of importance or offer any issue in any of the following: building his 318, swapping in a magnum, selecting and installing a transmission.

this is like worrying about if the cigarette lighter works in a car you're going to buy. a benign issue that is not pertinent to the matter at hand.
Don't blow a head gasket, I know that it's not a real important issue but it's still something that is part of what will have to be addressed eventually. I kinda think that you don't like it because you know that I'm right on this one. I've changed out many a third member in my time and or tire sizes and found that my speedometer didn't read correctly. If you don't want to believe me that's fine, simply call your local transmission or tire shop and they will tell you what I did