installing motor with headers on?

You won't be able to install from the top with the headers attached. Best you can do is lay them in the compartment, but even that's not the easiest thing to do as you'll have to hold them up somehow and they'll have to move more than once to clear things as the engine goes in past them.

The only way to install the engine with the headers attached is from the bottom with the K frame, just like the factory did.