The 3116 in your truck is most likely the MUI (mechanical unit injector) fuel system. The 3126 was offered in a MUI version but, mostly as equipment and marine engines. The 3126B was the HUEI (hydraulic electric unit injector) and uses and ECM (computer) the operate the fuel system. The B series also has three valves per cylinder and later became the 3126E. It now is call the C7 because it is 7.2 liter displacement.
It would take a lot of wire to install the HUEI version. If you have a MUI engine, it should bolt in. The only difference in the 3116 and 3126 is the 1mm larger bore in the cylinders.
I don't think the HUEI engines swap any parts except the crank and rods. Always ask you local CAT dealer for any surplus engines. You might get lucky and find a crate engine for a good deal.
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