Hi,
Not too clear about your tensioner slipping. Sure you are turning correct way to tension?
You don't really need any marks on the belt to fit it.
Assuming that you set the timing marks on camshaft pulleys correct before you removed the old belt, and camshaft and crankshaft locking tool is in place.
Without the right hand (non adjustable pulley) in place, and the top centre (adjustable pulley) set to it's minimum position, turn the crankshaft ANTI CLOCK just slightly, (about half width of a tooth). fit the belt on the crankshaft pulley and fit holding wedge.
Feed the belt up around bank 3 & 4 pulleys and under the top centre pulley and over the bank 1 & 2 pulleys, over the tensioner pulley.
You can't push the belt right onto the pulleys as you go, you have to push it a bit at a time onto each in turn, making sure the teeth are in the root of the pulley teeth. Just have a little tension on the tensioner at this stage.
You should be able to now fit the right hand pulley and nip up gently.
Now turn the crankshaft pulley clockwise to its correct timing position.
If you have the correct number of teeth between the crankshaft and number 4 pulley there should now be hardly ay slack movement in this part of the belt run. If there is any real slack then you have start again.
(before you remove any of the locking tools).
As long as all the belt teeth are in the root of all the pulley teeth, and you have some tension on the tensioner, you can now remove the locking tools. I found you have to sometimes just try moving the camshaft pulleys slightly to take the weight off the camshaft locking tools to pull them out.
Just check that the timing marks are approx correct, ie, that is within about 2 to 3 mm before you give the engine a few turns over to check timing with the setting tool.
IF you've got it right as if by magic the bank 3 & 4 will be spot on, and you will only need to adjust the top centre pulley to get banks 1 & 2 in correct position. Don't forget you need to move and hold the adjuster with a 30 mm ring spanner then tighten the centre bolt.
Set the tensioner at correct setting and recheck several times and double check you have tightened all the pulley fixing bolts.
I don't have any special mechanical genius, but was a piece of cake, spot on first time.
Let us know how you get on.
Roger