I tried to fit the dipstick and discovered that there isn't a way to install it once the headers are on and the motor is in the car. Luckily (for me) this is a mock up so I don't have to pull the engine, I just need to remember this for when the real motor goes in.
I can see why this dipstick wouldn't work with the cast Holley manifolds. It hugs the block and head pretty close. Maybe the OEM truck dipstick? It curves out and around the stock exhaust manifold.
There are some pictures of the dipstick installed in the below thread though. Not with the Holley headers but maybe it will help you.