Well, for good power there's no substitute for cubic inches (on a normally aspirated engine).
So, if it's not outside your budget I'd suggest doing something like this:
"stroker kit" consisting of a 4.25" stroke crank, forged rods, flat-top forged pistons
Your 6x-8 heads, prepped with good 7/16" screw in valve studs, fresh valve job, port matched to your intake.
block bored +.035 over (or whatever it takes to get good clean cylinders based on its current condition)
block zero-decked (this means shaving the top of the block as required to make the distance from the tops of the pistons to the top of the block exactly .000" - Pontiacs from the factory are usually around .020 "down")
All this will net you 461 cubic inches (or so, depending on how much overbore you need). If you add some thinner than stock head gaskets (.035 compressed thickness Cometic gaskets) you'll be at 9.2:1 compression with excellent quench characteristics -assuming your 6x-8 heads have the 'factory nominal' chamber sizes of 101 cc's (always measure Pontiac heads yourself because the do vary). Without going the stroker route, it'll be pretty much impossible to get your compression high enough on a 400 with those heads.
From there you pick a cam shaft and remainder of the valve train depending on what you want, your budget, and need for power brakes. I built my 461 using my numbers-matching 69 400 block, solid roller cam, ported Edelbrock aluminum heads. It's probably making somewhere around 520-550 hp on 93 octane and has run a best of 11.86 at 113 on the track. My cam though doesn't quite make enough vacuum to be reliable for power brakes (in my opinion, so I'm running a hydroboost brake system instead). The quench properties on my engine are now probably "fair" to "not so fair" (long story as to why).
When I broke it in on an engine dyno (with different heads than the Edelbrocks) it made just a hair's breadth under 500 hp, 540 lb. ft. of torque.
I'd recommend using reproduction "Ram Air" exhaust manifolds instead of headers. These manifolds won't make quite as much power as a good set of 4-tube headers, but what you'll save in headaches on installation/maintenance/etc. ought to more than make up for that on a street car. I'd run the factory cast iron intake manifold and a properly prepared QJet (everything port-matched to the heads, water crossover cut to separate it from the rest of the intake). On a low-rpm street cruiser that combination would be very hard to beat.
If you were to build a 461 with your existing block and heads, you'll spend "about" $1600 or so for the stroker kit (crank, rods, pistons, bearings), "about" $200 for the Cometic .035 head gaskets, "about" $800 for a good hydraulic flat-tappet cam and lifter kit. The rest would be whatever you spend on having the various components machined, gaskets/seals, and "extras" like missing parts/exhaust/ignition system/rocker arms/valve springs etc.
For all that, you'll have a reliable engine that will make boat-loads of torque at low rpm, peak horsepower probably somewhere around 400, peak torque probably somewhere around 460. It'll run great on 93 octane pump gas, make enough vacuum for power brakes, and be an absolute blast to drive.
How's that sound?
If you want to step it up from there, then you could go with a roller cam (a lot more expensive) ported aluminum heads (also $$$ if done right), "more" carb & intake, headers.... pretty soon you'd be making the same power as me, if not more.