For the speedo, one way around that issue is to get a late model 240 diff with an electronic sender in it, then wire it up to a 86 or 87 model 240 instrument cluster which has an electronic speedo in it.
Alternatively, there is also a motor and electronic sender unit commercially available that allows one to attach the motor unit to the existing mechanical speedo, then an electronic sender unit is attached to a late model 240 diff (or a sensor that reads the speed by tailshaft rotation, etc), which sends the correct numbers of pulses to the motor to drive the speedo motor. Only problem is that the sellers want a small fortune for it, and I'm not sure it includes a kit to get a signal from the tailshaft if you don't want to do a diff change.
http://www.abbott-tach.com/cablex.htm
A third alternative is getting a transmission rear case section which has the mechanical drive boss for the speedo in it, but the availability of it depends which type of auto trans you're going to use.
(Being in NSW where we have annual roadworthy inspections, I have to consider such things as a speedo drive for my highly modified car-in-planning, y'see).
