want FATTER tires

[bakerboys]

I have a lifted 85 k20 6 inch lift 37 inch tires 17 inch wheels

they only make 17 inch wheels 9 inch wide thats it

I want fatter tires so if i sell my current wheels which are new and my current tires which are new and $400.00 each toyo m/t

and then buy some 15 or 16 inch wheels that are 10 or 12 inch wide with some 38s or 40s
ARE there any modifications i would have to make for the rim to fit

Also I have 4.11 gears will that be enough gear for 38s or 40s

thanks