[AT] Dumb tractor hauler truck question

[charlie hill]

Well after 356,000 miles I have finally had to replace the last major 
component of my pickups drive train.   The  transmission was first to go at 
about 240,000.   Then I replaced the engine at 276,000 (before it let go) 
with a GM Goodwrench engine.

I did a bit of hauling over the weekend with no problem but on moday morning 
the occassional light roaring I was hearing from my rear end turned into a 
howl reminiscent of a worn out tractor transmission.  Turned out the carrier 
bearings were shot and 2 days and serveral hundred dollars later the truck 
now has a "new" (read salvage yard) rear end.

NOW THE QUESTION.   Since I do a fair amount of towing with this truck I 
decided to replace the factory orriginal 3:08 to 1 rear end with a 3:42 to 1 
limited slip.  NOW I need to get the speedometer to read accurately.   I 
know how to do that.  I just need to replace the sending unit with the 
corect one.  My question is where to buy the sending unit.  Is it a dealer 
only item or does anyone know of an aftermarket source.

If anyone knows of a possible source what I need is the adapter for a 700R4 
transmission with a 3:42 limited slip gear and 235 75R 15 tires.

Thanks

Charlie