I got a few real good peices of advice when i built my first pontiac 455 ho. First of all dont even think of building one.
Before i go further i want to say i love pontiac. If i had a good block i would put on in my c4 vette just to be wierd.So as you can imagine i did not take this good advice. Hell my 71 grand prix sj had its own website. It did work out big cubes it was fast as hell. I cant tell you how many people were suprised to see that 4000+ gp jump up and leave them stand still in a cloud of tire smoke at 50 mph.
Deliver pizzas until you can buy one turn key from butler. You will save thousands and end up with more power on pump gas friendly engine you will love everytime you drive the car.
Butler Performance - Specializing in Pontiac Engines Heads and Performance Parts
You can build a big chevy for 3k less. Nothing to sneeze at. And it will make more hp.
For the cash i spent at the machine shop and parts i could of had any corvette engine low miles ready to swap. It really is LS-whatever.
A gen one small block twin turbo setup with everything forged and computer controled. In the 1000 hp range is not out of the question. If you going to pay 7 k for a dirty block and a box of parts you can do anything you could imagine with a sbc.
That being said i know nothing about the 350 sep the 400 is more common and easy to lay your hands on. Hope you get it going. best way to build 455 is get parts for next to nothing from friends and the net over a few years when you can wait for the right deal to come along. They are out there just never when you need it.
Sorry for posting usless info that is not really related to your post. Just dont want to see anyone go down the same road i did. Butler rules... I talked to him before my build hes the one that gave me the advice.