What would you do?
 

I have no power what so ever on my rebuilt 2.4L "T" motor. I believe I did everything right as far as assembling the engine. What are some things I should check. Im talking a lot of power loss, estimate 30 to 40%. Feels like Im driving a 912.

Power???
 

Of course you need to do the obvious things such as a compression test on all 6, and then a leakdown test (that requires special tools, but it tells a lot), you might have your valves set too tight or loose, might have a bad ring in a cylinder... Ensure the timing is set correctly. There might be a leak in an intake gasket. Take a vacuum reading. If you have injectors, one might be clogged. A bad spark plug or so...
Your question lacks details for us to go on. Who did the rebuild, was there an upgrade done too? Etc Etc Etc. !!!!!

I'd check the timing first. If it's too far retarded, the engine will start like a dream but will lack any sense of power under load/acceleration etc. If it's not that the, as stated above, you're going to have to go about things in a methodical manner – check valve clearances next as it is easy to do them on the 'wrong' cycle. Good luck!