Speedometer/trans/differential question.

[Clemson88]

When I inquired about which speedometer to use on the diesel into a gas body I was persuaded that in order to get a proper reading on the speedo I'd have to use the speedo from the diesel car.

Both cars are early w126 and I wanted to use the differential from the gas setup because I like the gear ratio. Now I read on the thread about differentials that in order to use the gas differential I'll need to use the gas speedometer to get an accurate reading.

The diesel engine and transmission came from an 84 300SD and went into a 85 380SE body. The original differential got a yoke change to accommodate the diesel drive shaft.

Which speedometer will work? If neither with this setup what should I do to fix it short of changing the differential?