I thought of something else, because you mentioned 310 HP economy, I believe that governor might be set to an 310 HP @ 1800 rpm rating.

 Your old engine was a 2100 rpm rating, so if a person used the 310 engine and governor, you would probably need a 2100 rpm governor spring (inexpensive) inside that governor, too.

I'll tell you a story, and I'm friends with the CAT foreman in Omaha, and about 10-12 years ago, he mentioned that some truckers with older electronic engines(PEEC) set at 310 HP, were going to truck wrecking yards and buying used 400 HP ECMs (engine computers) and bolting them up to their 310 HP engines, and not changing anything else. He said the little 310 HP turbos lasted, quite a while (= actually spinning too fast), then when they did fail, the truckers would have it replaced with a 400 HP turbo part number, at that moment.

 I assume some of the other 310 HP parts probably weren't correct for 400 HP either, but according to him, they must be holding together with the amount of fuel going thru a 400 HP engine. 

I mention this because you might be getting "overly worried" about getting these engines set up, to CAT specs, that CAT had to use to pass emissions and get decent fuel economy numbers, BUT it doesn't mean some of these "matched components"(inside engine) can't handle some more HP, without engine damage.  

For me, I might be more interested in, what is the most inexpensive way, to get that "3ZJ" engine up to 350, 400 or 425 HP rating at 2100 rpm. {Meaning, which one of those 3 "CAT Factory" choices, used the same pistons, engine camshaft, and fuel injection pump camshaft as your 310 HP economy engine uses and the SAME base Timing spec. (big ticket items;$$$)}

 After that, then you look at fuel nozzle part number differences and turbo part number differences (= these are cheaper and easier to change).

I'm pretty sure you will automatically need to change, items such as Governor Spring(for 2100); timing advance spring (maybe), Torque spring and shims (inside governor, cheap); and reset fuel settings, for roughly 360 - 400 HP.

Honestly, the very cheapest route; is to just install a 2100 rpm governor spring, turn the two fuel screws out another 1.5 turns each, and set "high idle" (top engine rpms) to 2300 rpms, on the newer "3ZJ" 310 HP engine. (At least you know all the components, inside that engine, match for a 310 HP configuration)