Damn, wish we had a project like that when I was a boy scout... sure beats building fences... lol
Alrighty, so measuring for shock length. This is a little trickier. Assuming you building like a Stock Mini or some class that has to use the stock mounting points to figure out the shock length you will need to again cycle the suspension. This time though measure (front) from the upper shock mount to the lower shock mount with it both drooped and bumped, at this point you will know the collapsed and extended length of the shock that you will need, you can then look at the specs that the given shock manufacturer has and try to find one that will fit the needs. For the rear do the same, or if you can rework the mounts fit the longest shock you can get in there.
What suspension are you going with? Most manufacturers will tell you what size shock to run. If it is the stock suspension you will be very limited in the type, size, and brand shock will work anyway.
So what kind of truck are you guys planning anyway?
Imagine if sharks could walk on land... Scary.
1999 Long-Travel Ford Ranger XLT 4x4 4.0 4door
2007 F-150 5.4 FX4 SCREW