I'm working on this now myself. 3" makes it level to sagging. 4" has a bolt mount issue but would be level and 5" is rear high, but will (probably) level with a load.
If you are using the 105 springs and perches the top bolt mounting to the frame is an issue. Check measurements to see.
I've seen some with a spacer between the spring and axle (a prior post at the beginning of this thread talks about doing that) or as the OP of the thread, using the frame section in it's entirety, making it 5" higher. The 3 1/2" I believe makes the truck level with no load while a 5" space will make it rear high, but level out with a load. 4" makes the mounting bolt line up with the flange of the frame...not a great location.
I measured the springs and the deuce springs are the same 2.25" wide, same as the 105 springs, but the 105 has more leafs. I'm going to remove the three leafs that are at the bottom of the 105 spring pack and add new frame to attach to, not space the axle away from the spring. Note the 105 frame is 1/8" thick and the deuce has 1/4" thick frame.
I have a boomtruck that has reinforcement on the side of the frame for center mounted boom. Then I looked at the deuce and saw that I could do something similar on the outsides of the frame using 1/4" plate and having it made into an L, lining the top up with the top of the frame and the bottom then being 5" below the deuce frame. Some gusseting and additional bracing will be added, especially around the spring mounts, but the plate reinforcement will extend all the way to the back of the cab, helping to support a (smaller than HEMTT Grove) crane.
I'm ordering the steel with 13" flat, 3" flange, 8' long and 1/4" thick. The fab shop priced those 2 pieces at $200.
Hope this helps.