you are allowed 10% of wheel base, try not to go all the way,
also , make youre tunnel bigg enough so you dont have to get long ext and ratchets to get trans bolts out,
also dont want to go back to far on the motor might have to get into the windshield ?
as for how high./low the motor get , depends on where the rear sits, and angle of trans, never want the trans to go uphill, 0 or 2% down (really dont like 0), this is dictated by what ride height or stance you want to achieve?
my motor is moved back 7" less than 10%, but my motor is as low as i can go without the trans up hill, and my oil pan,
i cut up my k memeber is all i use is the bars i have an open area except for the rack/pinion,
good luck