Yeah, you just can't properly tune it until the exhaust is fully built.
That said, you should be able to at least get it close enough to work.
How did you switch the pointer and still have it line up? Did you make your own marks, or does your damper have the multiple markings for the different Ford setups? Or have you mixed and matched dampers from the different engines?
Sounds like you used the front cover and such from the 351, rather than the ones from your original 302, correct?
First, make sure that the idle speed screw is all the way out so that it's not even touching the lever arm. Then make sure that the choke mechanism has fully opened and released the high-idle cam past it's last notch. This is often why a carburetor won't idle down.
A rebuilt carb often has this trouble due to improper re-setting of the throttle blades. But new ones don't have this issue that I know of. Probably not impossible, but worth looking at all the other things first.
Is the PCV valve connected to one of the carburetor's large vacuum ports? Make sure it's not connected to a single runner in the intake manifold. Has to be to a common area.
The brake booster can be connected to the single runner and a vacuum tree at the back of the intake, but since the Edelbrock carb has two large ports (that correct?) you can also run the booster to the extra fitting.
When you moved the pointer, are you sure you nailed TDC perfectly? Any chance your initial advance is too far? What is it reading currently? Sometimes if you have your base timing set too high you won't be able to idle it down.
Also check for vacuum leaks all over the upper area of the engine. Lots of room for mistakes when changing a manifold and carburetor. But the timing and choke settings would be the first things to check I think.
Good luck.
Paul