hey just a couple of questions. since i put my new 5 speed and posi 373 in the speedo has been way off. besides cracking the trans open and changing the drive gear is there anything i can do. it reads 110 when im doing 75.

also since i swapped the 350 for the 3.1 my tach has been off, it reads a lot higher rpm than the engine is actually turning what can be done about that.

 

maybe swap over to a v8 cluster?

 

Swapping to a V8 cluster will fix the tach, or if you search, you can find a way to modify your stock V6 tach to work with a V8.
The only way to 'fix' your speedo is to get the proper gears and install them.