I have just came across a tire the size is 33x13.50x15 Nitto Mud Grappler. I am wondering if they will fit with the stock ride height? It seems like I have plenty of clearance for a 33 inch tire (in theory).
Yes and no.
It's a close call and really depends on how much sag you have in your front springs.
These diesels are heavy and these springs are old.
I've had 33x13.5's on a civy K-5.
I needed to remove the full length factory running boards (these trucks don't have).
Without the running boards the only clearance issue was on a heavy deflection, full steering, off-road the tire could actually flex far enough (low tire pressure) to snag the grease boot on one of the steering joints.
So the simple answer is yes, they'll fit without serious issues.
Add the heavy engine and years of potential spring sag and it's likely you could have minor clearance issues with the rear fender.(I should say rear of the front fender)
My civy K-5 was old to ('78 ) but it was a much lighter inline six.