Giovanni, the tallest gears that you will be able to get for the Eaton rear end on the 3600 are 4.10. You can find them if you look hard and are patient, but they are scarce. '67 to '72 Chevy C20 and C30 trucks with 350's and automatic transmissions usually came with them. The 4.10 pumpkin is a relatively easy direct bolt-in to replace your current gears.
The 4.10 will increase your actual speed at the same RPM by 10%. There is a Tech Tip on how to do it.
Larger diameter tires will give you an additional boost.
You would need to change rear ends to get the gear ratios that you want.
Matt
