I'm putting the rest of the parts on the 455, new melling pump and pick up, I've read 3/8" off the pan, so I taped a 3/8" rod to the pickup, to make sure pickup isn't hitting pan, the pan didn't sit all the way down, I lightly pushed pan and it layed flat, my question is, should the screen be level? With it being crooked I will be 3/8" on 1 side and over 3/8" on the other side

 

Just replaced the pump on my 1968 455. The original was within 1/8" of the pan so that is what I set the replcement to. At this height it was very close to level from side-to-side, but it was definatly sloped down towards the front. Same as the original.

 

You want the pickup to stay submerged so it doesn't suck air, but also far away enough from the pan so it doesn't just suck itself onto the pan. Neither of those are precise sciences.

Pickups like the stock style that have a cup shaped protrusion under the screen can be quite close to the pan. That cup helps prevent the pickup staving against the pan. Pickups like most aftermarket ones that are just a box with a screen on the bottom need to be a bit away from the pan so they don't lock up.

Usually folks put a block of clay on the pickup, install the pan, then measure the clay to see how much room they have, or directly measure the pan depth and the pickup distance from the mounting rails.

I bet you're fine with keeping the pickup level with something around 3/8" clearance. If it's a press-in, just make sure it's solidly mounted as some can vibrate out if poorly installed. Some folks tack weld those pickups in place as a fail-safe.