new / want advice

I have a 66 pickup with a 250 3 on the tree my problem is engine revs to high when running down the freeway just want advice on how to solve this problem what would be the easiest solution thank you guys for your time  

Is the clutch slipping, or in good working order?  If there is no problem there, the factor determining RPM is the overall gearing of the truck.  Your options are different rear end gear, or a simpler solution could be as easy as a different tire height (taller tire) to change the overall ratio.  What is the RPM at highway speeds?  Do you know what gears you have in the truck?  What RPM are you trying to attain?  

The clutch is fine the truck runs really good, the tires are just stock tires,I dont know what RPM im running at, all I know is when im running down the highway it sounds like I need another gear. I have been told to find an overdrive transmission or just change the rearend out with another, its just I really dont know what rear end would work, and with the overdrive is it something that just hooks up to my original trans or do I have to swap out the whole tranny? I just need a lower RPM going down the highway. thank you for your response  

I paid about $500 for parts and labor to have a shop install new gears in my '02 Mustang a while back.  The results were good and I was happy with the way the car drove after that.  Sounds like you need some taller freeway gears to bring down the RPMs.  

Take the inspection cover off the rearend and calculate the rearend ratio.  Its more than likey it is going to be a 4.10 or 4.11 ratio being that the truck has the 3 three speed manual.  To calculate count the number of teeth on the ring gear divide by the number of teeth on the pinion gear.  Put like a 3.08 rearend ratio in the truck and it should cut the noise down.