Re: Gear Ratio
The 6.0 and most 7.3's can be done by any Ford dealer with the right ECU tool, or by some aftermarket tuners.
Autoenginuity software will re-program some ford speedos in the exact same manor as the Ford ECU tools. (IE, no new tune/ECU program) Last time I checked (~2years ago) they only supported the 6.0 on the truck, not van, not sure on the 7.3
Have no idea on the Gas engines.
I did find a thread somewhere that described a process where some 7.3 owners had done it by following some simple instructions found on the net. But I can;t find it now.
It was something like shorting 2 pins under the dash, then pressing the odo buttons to get into some programming mode. This apparently worked on a particular a ford speedo module that they put on a number of different models, all pre ABS I believe. Might be worth a few hours searching.