When searching for HP, I generally look under the bed or in that kitchen draw that has "stuff" in it that you don't want to chuck out, but might use, along with the rounded off tipped phillips screw driver, sloppy pliers, dull razor knife, and flashlight that doesn't work.
Looks like you have about 9.5:1 compression with your heads and have selected the Comp Cams 041H cam.
I could not find if the .516" lift is with 1.5 ratio rockers or the 1.65 ratio? Several things come to mind. Are you pulling your heads or doing this with heads on? With the .516" lift, you may or may not need to open up the pushrod holes at the top of the cylinders where the pushrods go through - there are 4 of them that are round holes. With a higher lift cam as this one, the pushrod can hit/bind/bend on the side of the hole at maximum lift - more so with the 1.65 rockers.
"I did that once. I didn't elongate the pushrod holes and ran 1:65 rockers. I used the Harland Sharp setup. I wish I did elongate the holes because I wound up bending some pushrods. Had to replace the pushrods plus spend another x amount on gaskets after pulling the heads off, plus all the labor once again. I wouldn't recommend doing it on the engine as metal in the oil basically can destroy an engine. You only have to lay the holes back approx. 1/8" or so about 3/4" down to get rid of the material. It's not hard at all just takes a bit of time depending on how "sharp" your burr is."
With that cam, you will lose some bottom end power. Often recommended with the 041 cam in a 455 is the Rhoades Lifter. You can get these with the Super Groove Lube which can help in cam lobe oiling. Used the Original Rhoades Lifters (no Super Groove) in my brother's 360 Mopar build, 9.5 compression, 284 duration cam, .491" lift. The engine idles like a stocker, good vacuum to operate the power brakes, and pulls hard. I can't really hear the lifter noise that most seem to mention unless the hood is raised and I put my head near the rocker covers - sounds like solids. It has been several years since I rebuilt it and I think he has about 20K on the engine. You can read more here: Products
With the higher lift, you will want to use matching springs - possibly retainers depending on your source on the valve springs. You will note that CC states the heads must be cut for their springs, probably to get the correct installed height. The RA IV heads used a taller valve for the higher lift cam. Not sure what the 455HO heads used, so you want to make sure that you have the proper clearance between the spring coils at full lift as well as the bottom of the spring retainer to the top of the valve guide. Again, you can get spring retainers that are taller then stock that could eliminate any need for head machining and give you plenty of clearances. So this needs to be checked.
You may also want to opt for better/stronger pushrods so you minimize deflection on the higher lift.
I would not personally use the Holley double pumper on the street unless you have some 3.90 - 4.33 gears. Pontiacs are heavy cars. A vacuum secondary is the way to go. Best investment is the factory Q-jet manifold and Q-jet. Save your money on the Edelbrock and spend it on a rebuilt Q-jet.
Exhaust system is right on.
You will need the long slot rocker arms. If you go aftermarket you won't have any problem as they seem to all be long slot. If the short slot rockers are used, you will bind the rockers up at higher lifts and bend/break things.
Change out the bottle neck 3/8" screw-in rocker arm studs to Chevy Big Block 7/16" rocker Arm studs or you can snap them off with the higher lifts. I used the ARP brand. This will also require you to use some form of Poly lock as the factory type lock nuts will back off. The poly locks are positive locking and won't back off. Now if you use the poly locks, I am not sure how they would adjust with the Rhoades Lifters, but they do have a set that get adjusted just like solids, so this might be the way to go if you use them. You can check around on the FAQ sections which might answer that one.
I think that should cover it, but may have missed something......oh wait, its in the draw next to the loaf of bread twisty ties.