I recently purchased a 65 c20 with I6 and the casting number on the engine says its the 230. My tranny is a 4 speed with the granny first

Not sure what rear axle gears I have but they feel like 373 or 411's and i currently have 15 inch wheels.
Currently, going 55mph the truck is buzzin very high rpms.
What would be the best and or cheapest option to make? I'm only looking to go 70 max and that would be on a rare occasion.
I'm going to use the truck as a cruiser only. Quick take off is not necessary either.
Can you guys give me options and maybe what vehicle's I would want to search for to make it happen?
Everyone's advice is welcome!
Thanks ahead of time!