My buddy has an 85 chevy p/u and hes pretty redneck and doesnt have a computer so he wanted me to find out for him an answer to this question.

How big of a suspension lift can you put on an 85 chevy pick up before you have to lengthen the driveshaft?

He thinks it might be 4'' or 6'' does anyone know?

up to 6'' you do not have to lenghthen your driveshafts

8'' lift will fit that truck just fine.
10'' and up you will have to lengthen
the driveshaft.

Running a 4'' lift requires a 4'' steering arm & extended brake lines. The only extra mods needed to jump to a 6'' lift is a 2'' pitman arm & a front driveshaft spacer. That is what l am running & it works great.
Tell ur buddy that he will regret going with a 4''. Its not much more trouble to run a 6''.

l run a 76 3/4ton chevy with a 6in Skyjacker & It has the same front end distance as the full size blazers.

6in & 35 is r a good setup. You can also run 36 is if u want also with no fender cutting.

Plus over a six inch u run into steering headaches, needing to eventually go to the crossover steering for things to work smoothly.

It really depends on what the truck will be used for. If its a shiny ride that stays on the Highway except for an occasional mud-hole, u can use the stock shaft up to 4''. Stock shaft with 6'' inches of lift is not recommended. It would probably vibrate terribly. Now if you're gonna do some serious Wheelin, u r gonna want to make a new shaft ANYTIME u change ur trucks height. If u get in a serious twist on the trail, u WILL lose ur stock shaft if you're running 4'' of lift.