I have a 1991 crew cab and i was wondering if a 6 inch lift will be effected by the wieght of the 6.2 diesel... im sure it wont be a true 6 inches but how much will it effect it?

I have a 1991 crew cab and i was wondering if a 6 inch lift will be effected by the wieght of the 6.2 diesel... im sure it wont be a true 6 inches but how much will it effect it?
Most lifts use the stock torsion bars (springs), so if it the truck was made for a 6.2 and has the factory torsion bars that came with the 6.2, then I wouldn't expect a problem. The only issue might be is that the bars could have relaxed a bit over time, since they are 18 years old. You may still be able to crank them a bit to compensate.

A 1991 Crew Cab would have the Dana 60 and leaf springs.

The 6.2 is not much heavier than a 454 so as long as it is for a 3500 series it should be fine

Some lifts are gas or diesel specific. Due to the weight as you mention.