[crushers]

for every 1" of lift you can 2" bigger tires (1/2 the tire is bleow the axle, right?)

so if you go 6" lift you can go 12" bigger rubber with the same clearence for travel same as stock. this does not allow for circumfrance of the wheel rubbing on fenders or frame when turing.
ie. if your stock truck has 2" of travel (and most do from factory, generic statement) then 6" of lift will allow you 12" bigger tire with the same 2" of travel.
stock tires on most crusiers are 29" (31" on the 80 series)
so if he has 12" of lift he can run 24" bigger tire so 29 + 24 = 53" tall tire with the same travel as a s stock rig. (of course you will need to trim the fenders to allow for circumfrance interference)

me, i like low and stable so sheet metal becomes sacrificial when it comes to fitting larger tires.
so for 37" tires you want (37-29=8/2=4) a full 4" of lift. you could run a bit more travel by adding an inch or 2, so if you do a SOA (4"-5") you would be fine for 37" tires.

12" is WAY too excessive in my books and your COG will be the shits.

but then that is just my opinion.