Sorry to hear about your injector troubles, I'll be doing that in a couple of days & hope I'm luckier than you. I went through the same thing with my intake manifold a couple of years ago with my truck, it had about a 1/4" of sludge on the bottom. I couldn't live with it, so I decided to take the manifold off. I'm really glad I did because I discovered a couple of things when I did; First, some ***** matter for brains had punched a couple of holes in the valley cover (I guess to get the valley to drain- the drain hole at the back of the block was hopelessly closed). Second, it was obvious that the tube going up from the valley cover to the cdr needed to be replaced (I didn't really see the problem with the manifold in place). Another thing is the gasket and valley cover are one piece, & the valley cover includes baffles that slow down the flow of oil to your cdr, I'm guessing having these baffles function properly is critical to cdr function. My baffles were full of all manner of nasty crud.
I made sure I had a shop vac with a small tip to clean out anything that might have loosened up & gone into the ports. I also had a mirror & good light to look into the ports. The truth is that I needn't have worried much about this, most of the sludge was trapped at the bottom of the manifold, only a film of oil made it to the ports in the head. If you do pull the manifold...... BE SURE TO DRAIN THE COOLANT FROM THE BLOCK!!!! BE SURE TO DRAIN THE COOLANT FROM THE BLOCK!!!! BE SURE TO DRAIN THE COOLANT FROM THE BLOCK!!!! BE SURE TO DRAIN THE COOLANT FROM THE BLOCK!!!!BE SURE TO DRAIN THE COOLANT FROM THE BLOCK!!!!