If you don't have $3000 or so for a set of new alum heads with all the trimmings, there are cheaper alternatives. If you are just building a street car, you probably are not shooting for 500hp +.
If you would settle for a 400 with about 350hp, you can use the cheaper 6x heads. A good pair of cores run about $200. Then you can have a GOOD machine shop rig 'em up with stainless valves, and a good 3 angle valve job. Don't need any porting or 1.77 exhaust valves.
The 6x heads already have hardened seats, so as long as you don't let the machine shop talk you into 1.77 exhaust valves, they'll be fine with unleaded pump gas. But if they cut the seats for 1.77 valves, there is a possibility that they could cut thru the hardened part and into softer metal.
With these heads, you can either shave some off the chamber and intake side of the heads, to increase compression, and retain good intake fit. Or you can just surface the heads, and use thin Cometic head gaskets to raise the CR. And then run one of the steep ramp cams, like the Lunati Voodoo series. These cams increase cylinder pressure, and raise what is referred to as dynamic compression, in otherwise low compression engines.
So, I figure you can make somewhere between 325 & 350hp with about 8.5 or so CR. If you cut the heads down a bit more, use thin gaskets, and zero deck the block, to get the CR up to around 9-9.25 CR, you can easily make between 350-375hp, with the correct combo of parts.
Now if you want over 400hp and 500ft lbs of torque, for some tire smokin power, you can go with a 4.25 stroker assembly from Butler.
Or, if you don't have access to a good Pontiac machine shop, you can buy either a 400, 455, or 400 block 4.25" stroker, crate motor, with 6x heads, for between 4000 & $5000 or so, from Len Williams. The entire 400 engine is not much more than a good set of ported alum heads. And he can add forged rods and other stuff, if you prefer. The 455 with an Eagle crank and rods is only $4500 + shipping.
400 Long Block
455 Long Block
455 Short Block