My 2013 GLK250 wont rev above 2500rpm ?

Try changing your fuel filter, if that does not fix your issue I would have the catalytic converter changed.

Plugged catalytic converter

when your car when you're at 2500 rpm that means that you need new transmission motor mounts or come on

MAP sensor may be going bad letting it over fuel the engine causing black smoke and bogging RPMs .

May not be getting enough fuel to the engine

A CLEVELAND ENGINE IN A FAIRMONT????HMMMMMMMMMMMM....SOUNDS LIKE THE PUMP IS GOING BAD. DO YOURSELF A FAVOR AND JUK THE FMX AND GET A GOOD USED OR REBUILT C-6 INSTEAD.

Crank sensor or engine speed sensor is faulty.

Fault:
Irregular power loss on accelerating
The speed of the engine is limited till 2500rpm
Frequent loss of crankshaft position sensor signal

Cause:
It is caused due to leaking from thermostat housing into crankshaft position sensor harness multi-plug

Remedy:
Check and clean the crankshaft position sensor harness multi-plug contacts. Rectify the engine coolant leak from the thermostat housing if found. Take a note of the engine coolant level and monitor it for changes, showing a leak is indeed occurring.

remove the aircleaner intake hose and, check the intake air control flap for free movement

Being it is a manual transmission and the engine was removed, it is likely the clutch and pressure plate not releasing completely at high rpms.

run the fault codes
could be a transmission actuator problem as it senses vacuum changes when moving the accelerator and tries to change ratios ( 2000 to 2500 rpm indicates a ratio change which is the speed increase momentarily until the Transmission range sensor puts it back in the correct ratio for the road speed
check the wiring to the transmission
it is a sensor problem not a transmission failing