They are right as far as the 4.0 or 5.0 but I see what you mean about the 2.9 as well. My 87 Ranger is in its final stages of being rebuilt. I have tried to upgrade everything I could on the way.
Some of these may not necessarily add power but are stronger, more efficient or just a safer alternative.
I installed a high volume oil pump - part number M-328 (better oiling)
Bigger WIX oil filter for additional 1/2 qt of oil -part number 51773 (better filtration, cooler oil temps)
Radiator from 1994 Explorer with auto transmission (Twice as thick as 2.9 radiator, more cooling)
Clutch, Flywheel, and Starter from 94 explorer with manual transmission (Much more surface area, will hold more torque)
port matched throttle body, intake plenum, intake manifold, and heads (better flow means more air)
180* thermostat and plenum spacers will drop incoming air temp (for each 5* or so you drop the incoming air temp, you increase hp by 1%. So a 200hp engine could gain 5hp by dropping intake temp by 15* from stock 195* to 180* for about 5 bucks)
cold air intake with K&N air filter (Same concept as above)
Flowmaster exhaust and hi flow cat
electric fan (Less drag on engine)
timing set at 12* btdc
cap and rotor with brass terminals
I am currently looking into free floating rockers and see if anyone has already made a set. The only internal mod I havn't found yet.
These are things I have not done that should increase power as well...
adjustable fuel pressure regulator
performance coil or ignition box
5.0 throttle body conversion
The 90' ranger 2.9 throttle body is really small compared to the 86 2.9 throttle body, that's a quick, easy bolt on for a bigger throttle body.