I'm using a inline pro 3mm headgasket for a week now. Since the head is lifted off the block 3mm's does this stress or stretch the timing belt? Because it's a lot harder to put the belt on now, this is on a b18c1.

 

putting your timing belt on should be harder with 3 more mm to overcome. there shouldn't be a problem thought since you are using a tensioner.

 

How's the tension on the timing belt with that thick of a headgasket? Did you adjust the cam gears to compensate for it?

 

Tension on the belt is the same, just harder to put on. I haven't timed it yet.