a 10 extends by 10"
an 18 extends by 18"
your suspension will only raise by the travel that is built into it.
eg. a g body rear may have built in travel to take a 12".........putting 18's in it wont make it raise higher unless you alter the link setup aswell
your question is to general......so much depends on what ride etc etc.............
to give a general answer...........
6" fronts and 8" rears will more or less lift your ride to full suspension lift if you use about 4 turns of stock coils...not "mad lift", just as high as GM or whoever designed it to go.
if you want big three wheels and/or "mad lift" you will nee to do a lot of mods to the suspension and a lot of strengthening
i am stripping a frame right now, in order to have it sand blasted and then off to be reinforced. do i need to extend my rear and lower trailing arms? does anyone know if i would "box" them in or just reinforce them on the 3 sides? one last thing i was looking at getting some drop spindles, would that be too low for the car it had 13's and it would scrape on some speed bumps thats why i want to put switches. The car is a 84 buick regal. what size cylinders would i need? thanks