Should I go 383 stroker or 455 big block

Just another added piece of info...
the rear of that car is a 10bolt 8.5"
you can go with a set of 3.42's to make it a bit snappy and still be able to hit the highway.
the standard rear as is will hold to my previous post.
What is the tire size you are planning to run?
As for the overdrive trans...Just remember to put in that 12volt switch to turn off the overdrive.
this is allows you to stomp on the gas and go, turn the switch back and you can cruise at desired speed.
Remember going with a 200-or-700 you have to change the driveshaft length.
Have the th350 built with good hard parts and it can hold a solid 500hp.
I have a th350 with LENCO internals and a hughes 2500stall converter.  I spent 1800 on the package. *i may have an advantage, LENCO is 9 blocks from me*  

Bracketry for the front accessory drive on a Big Block Chevy is available NEW just about anywhere and the 350 trans has the same bolt pattern. The distributor and ignition even interchanges. Headers are abundant for that chassis/engine combination.
Get a 454 Chevy from a 90's truck and go onward from there; there is NO substitute for cubic inches.  

i WOULD SUGGEST THE 455 AS IT WILL BOLT RIGHT INTO A PONTIAC WITH NO HASSLE WHATSOEVER. THE 455 IS A BIG STRONG ENGINE THAT RUNS OUT REALLY WELL.  

Yep, the 455 is the way to go.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

FIRST of all, don't build a car without an idea of what you want to use it for. Unless dumping money down a drain appeals to you.  Do you want smiles per gallon, or miles per gallon?  Is this a weekend toy or a daily driver?  

SECOND, make it safe.  A 500hp big block is useless if you don't dump the money into brakes that make it stop, get that front end fixed, buy a new radiator (the one you have now is loaded w/ the scale and rust and garbage from your 350--you want that to go into your new fresh engine?), flush out the fuel tank, get 'lines big enough to feed the engine you want, and so on.  Price those 4 speed automatics, before you decide how much to spend on your engine.  The result may be that you cut back on what you do with the engine--or invest in a strong bottom end, then invest in good heads and intake once you save up more cash.

THREE, price a GM 385cid engine.  It might be cheaper than stroking out the 350 you have.  See the budgeting comment above.  A Pontiac 455 is hard to find these days, but 400's, which have different main sizes, can be stroked.  But if you can find a 455, sheesh, there are recipes to make 500'lb just w/ stock parts.  That'll pop your spot welds easily enough.  Like any non-Chevy engine, you just have to research and know what works.  I got 96 heads (1971 2bbl 400) for $425 and 6X heads for $75...in other words, if you know what's what and the other guy doesn't, you can make out like a bandit with Pontiacs and Oldsmobiles--pun intended.

FOUR too bad about the Q-jet, its good if you understand it.  If that 350 has good compression seal, honestly, you could just throw some corvette/Z28 parts at it, maybe Vortec heads, rebuild the Qjet and get headers and an aftermarket intake.  They could make 300hp, 370 with 11:1 compression with just stock parts.  Those 2 bolt mains can last to 500hp w/ studs.  But with 300hp, maybe a nitrous kit if necessary, and gears in a car of that weight? You could be satisfied until you research Pontiacs and find the parts you need to get to where you want.  

I got a 455 in a gand prix and I love it my uncle has a 383 stroker in a 1967 chevy2 nova ss and my 4800 pound car is holding its own out there against that and all these 5.0 bustangs I call them bustangs cause they cant keep up but it cost some money to build a 455 but thats the way i'd go.  

Yep! I agree Pontiac 455.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

I just got my grand prix going again I was having a problem with getting her started but after tearing her down and finding out that the gear on the distributor and the gear on the cam was tore up I thought to myself lets redo the whole top end and while I had the money I got a 9inch stall converter and 373 rear end gears and man she hooks it now I droped about $3,200 in her just on cam,lifters,springs valves,mallory distributor,converter,ring&pinon gear,intake manifold,carb.,machine shop,elbow grease,and all the above.It's alot of fun when it's for fun but dont get down in the dumps when u see those $ numbers going up,because in the end it's all fun just remember that if U drive it HARD U will be under that hood soon like me! HA HA my wife hates it I spend too much money and time on my other girl(455)HA HA!