Old big block V8s with cast iron heads weighed a lot more than the Powertech 4.7. There may be kits that will work for many applications both old and new. It might be good to get confirmation from a manufacturer about your specific use before buying their kit.

 

My guess is its not the motor but the bigger transmission and rear axle that's heavier?

 

Have a look at this - http://wjjeeps.com/dimensions.htm - the V8 vehicles run a little bit heavier than the sixes but overall not by much. You're looking at these differences divided by four springs and there is something like 25-40 lbs difference on each spring - they will obviously sit lower as there is more weight on them. The question is by how much? I think the answer will be 'not a lot'.

IRO suggest front spring rates of 200 lbs per inch for a 2" lift spring. My old stock front springs measure about 13 inches so I guess a 2" lift spring would be 15", 40 lbs of additional weight spread over the 15" is not going to make a lot of difference in the way the vehicle sits, but for consumer regulations I imagine manufacturers have to state that the actual lift may be less - I reckon you'd do well to measure the difference if you put the same springs on both a 6 and 8 cylinder vehicle and sat them side by side on a level piece of ground.

 

If I had it to do over I'd get 1-inch higher springs for the front. I just added a 3/4 spacer to the front to level it out, and still. This with 3" IRO springs.