I Cannot Turn The Engine
 

So I am doing my first valve adjustment/general service. The Kokeln intercooler is off and when trying to turn the engine by the fan nut the belt just slips and crankshaft is not moving. The nut on the crankshaft pulley does not move it - rock solid.
I also tried to move the engine by turning rear wheels in 5th gear (915 box) - nothing.

Is there any trick here? would removing spark plugs help?

The engine is modified = 3.4 Mahle P&Cs 7.5 compression, AASCO High Performance Valve job with springs and titanium retainers, SC cams etc. Does it make a difference?

Please help.

Hmmm, if the engine ran fine prior to you doing the valve adjustment, I'm guessing your valves are timed, correctly, so there shouldn't be any valve train encounters of the third kind occurring. Are you able to turn the crank nut the opposite direction, a little, from the way you were trying to turn it? If you can, turn it a bit until it wants to stop. Without trying to force it, does it just not want to go any further after a certain point? I'm just afraid the valves are touching the pistons, for some reason...Or maybe you just have great compression!

Yes, you can remove the spark plugs to turn the engine over fairly easily by hand. Once you remove them, you can test for any contact, much easier, but I doubt this is happening.

The engine run perfectly just a week ago just before I put the car on the jackstands.
Can I safely nudge it using the starter?

Pull the plugs, it makes positioning each piston at TDC so much easier.

You have not set the clearance yet, correct?

I would probably remove the spark plugs and try turning the crank then, just to make sure you have no valve train clearance issues from something you have done...

Unless you have changed them, recently, you probably need new plugs, anyways.

Put a Box Wrench on fan pulley nut, apply pressure to right or left side of pulley belt. Crank pulley should turn.

^^^^ This. I put a torque wrench because its nice and long, and push the alternator belt in a direction perpendicular to the belt. I do this on the right side of the pulley, between the alternator pulley and the crank pulley, but closer to the alternator pulley.

^^^^ That's it, just push on the slack side of the belt with your hand that is not tuning the pully, it will not slip.

I usually pull the plugs and the rear wheels with it up on jackstands. Then put a broomstick between the lugs and the floor on one side, with the car in 2nd I use another to turn the other wheel in the direction of travel. For some reason I find it easier than turning at the fan pulley.

when turning the fan nut, take your other hand and put pressure on the right side fan belt pushing it in towards the center.....I usually take my right hand press my thumb on the end of the wrench to keep it on the fan nut, and then just press on the belt with my fingers, pressing the belt inward.