first,
start with a track width of the donor. the wheel base can be changed but the track width is what the front suspension is set up to run properly with. if you need to change out wheels for a different offset, to acomodate the wheels to fender relationship, then try to do this as minimally from stock as possible. the tire size is also a factor with this. google independent front suspension geometry for a bit more info.
do a measurement on the width of the cab from the donor as well. compare to your existing cab. it may be easier to swap the firewall and floor from the donor which will already have the cab mounts etc. you can use the existing donor seats, console and seat belts etc if it fits well enough
next,
check out the contours of the frame of the donor. how much bump up over the axle do you have because this will affect the floor height of the inside of the box.
next decide what you want to do for a gage and wiring package. you can use the existing dash cluster from the donor if you are handy with electrical.
next,
do a drawing of the old cab on the new frame and see what the height will look like and draw in some wheels etc. it will be easier for a swap if you don't want the truck sitting really low. the lower you go with the cab over the wheels the more chance of tire interference when going over bumps with the wheels turned, like out of a parking lot. we do that all the time so check it closely.
next,
if you like what you see and want to start, take lots of pics of the disassembly of both units. when you remove a bolt install it back in the hole it came from when the parts are off for storage. that way you will know how it went together. on the donor measure stuff like dash width, dash height from floor, width across doors, dimension from floor of truck to top of dash. when you have the donor in the shop and are ready to start taking it apart remove the rear shock bolts and fab up a steel bar that can go in place of the rear shocks. lock the suspension at the ride height of the stock truck before you take it apart. that is the height the truck suspension was made to run at. do this front and rear. the front can easily be locked using a link from the frame to the stabilizer bar mount on the lower control arm.
I am doing a 57 gmc onto an 04 awd gmc envoy. pm me with your email address if you want some more info on the troubles I ran into and how I overcame them. it is not a job for the faint of heart for sure.
an assembly manual for the old truck could help because it will likely have a blueprint style of diagram with frame dimensions etc. you may be able to contact a gm dealer for the same sort of diagram for the donor except the dimensions are a bit different because they are measured differently due to the method assembly at the plant. the gm heritage center may also be of help for both units as well as the old car manuals project online. you can also try the gm upfitter site and see if your donor truck is listed. they have frame diagrams.
http://chevy.oldcarmanualproject.com...to/49index.htm