IMO it's absolutely essential to manually rotate the crank at least 2 full turns and then recheck timing after doing any belt/chain timing job. Not as critical with a fully gear driven drivetrain as you're not dealing with tensioners etc. 
It's quite easy to have a belt slightly out of alignment which will drop tension when it runs - potentially jumping a tooth etc. 
I've had some setups that we're sort of half a tooth out and the only way to get correct timing was to set them up what seemed like a tooth advanced. Once it was all loaded up  after rotating everything was correct. Just doing it without rotating by hand after would have resulted in it being a tooth retarded. 
Hopefully you haven't messed it up completely and bent pushrods or damaged rockers. I'd be pulling the rocker cover at this point to check in case you need to order parts. Good thing with a 300tdi you're unlikely to damage valves/pistons. 
Good luck 
Steve
				
			 
			
		 
			
				
			
			
				1985 County - Isuzu 4bd1 with HX30W turbo, LT95, 255/85-16 KM2's
1988 120 with rust and potential
1999 300tdi 130 single cab - "stock as bro"
2003 D2a Td5 - the boss's daily drive
			
			
		 
	
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