There are advantages to both the 4.21" & 4.25" stroker assemblies. Most agree that, for street use, the 4.21" stroke is more than adequate, to make 500hp or more. It uses the Pontiac 6.625" rod length, & Pontiac size rod journals & pin size. The 4.25" assembly uses rods with BBC specs on each end, usually in either 6.7" or 6.8" length. Some say the longer rods have advantages. But, they are also slightly heavier.
I've added up the cost of buying the parts separately vs buying a complete balanced assembly from Butler. I have found that there is very little to be saved.
The one advantage I do see is the option to buy pistons made of the 4032 material, rather than the 2618 material used in Ross pistons & others. The 2618 material is used for racing pistons. It expands more than the 4032 material. Therefore, many consider pistons made of the 4032 material a better choice for street driven vehicles, since the engine can be built with less piston to cylinder wall clearance.
Auto Tec brand pistons are made of this 4032 material. The difference is explained on this home page.
http://racetecpistons.com/pages/autotec.phpAuto Tec has shelf pistons listed for Pontiac stroker assemblies. And, it is said that they will move the pin location, for no additional charge. This allows the builder to achieve zero or near zero deck height, without cutting as much material off the block deck.
https://shanonsengineering.com/collections/ebay-motors-parts-accessories-car-truck-parts-engines-components-pistons-rings-rods-parts/products/auto-tec-small-block-pontiac-400-428-455-flat-top-pistonsNow, if you choose to buy the assembly parts separately, you will have to choose the crank, rods, pistons, rings, rod bearings & main bearings. I am not currently aware of any Pontiac shop or engine builder who advertises a stroker assembly which uses the Auto Tec pistons.
About the intake: With 455 or more cubic inches, it has been proven that a single plane intake works great, even for street use. If you plan to use a square bore carb, you can use a Torker 2 intake.
If you wanna use a Q-jet, you can use a Holley Street Dominator intake, or the Chinese single plane, which is said to be copied after the HSD, but without the exhaust crossover.
A cam similar to this one should get you up to around 500hp or more, and still be very streetable.
https://butlerperformance.com/i-29361765-butler-comp-pontiac-custom-grind-billet-hydraulic-roller-camshaft-cca-3316-3317-hr114.html?ref=category:1272239 The "Stump Puller" cam made over 500hp with ported #11 iron heads.
http://www.sdperformance.com/viewProduct.php?productID=1815So, with good alum heads, you should be able to get up near 500hp, with this Butler cam. I see absolutely no reason to run .600 lift in a mostly street driven car. To me, it just puts more strain on the entire valve train. But, if max power is the goal, then a huge, barely streetable cam is called for.
https://butlerperformance.com/i-29361022-butler-comp-pontiac-custom-grind-billet-hydraulic-roller-camshaft-cca-3315-3316-hr114.html?ref=category:1272239https://butlerperformance.com/i-24452926-butler-performance-edelbrock-custom-87cc-290cfm-aluminum-d-port-pontiac-cylinder-headspairbpi-87cc-edl-dport-290.html?ref=category:1287735KRE also has roller cams similar to these above. Check out the HR230, HR236, & HR242.
http://www.krepower.com/Pontiac%20Cams%20and%20Parts.htmCheck out the 575hp KRE engine at this link. It sounds similar to what you want. The High Port heads flow about 330cfm, right out of the box. My engine guy built a 400 block 4.5" stroker, using unported HP heads, which made 755hp.
http://www.krepower.com/Pontiac%20Street%20Engines.htmhttp://www.krepower.com/Pontiac%20Aluminum%20High%20Port%20Cylinder%20Heads.htm"...I was told that I would have alittle trouble running pump gas if I wanted that power because the compression would be pretty high..."
With all due respect, if you have a budget big enuff to build a 600hp Pontiac engine, then the small cost of adding in some Torco Accelerator octane booster to your pump gas shouldn't be a problem at all. IMO
https://www.torcousa.com/torco_product/un_acce.htmlhttps://www.amazon.com/Torco-Octane-Booster-Quarts-Accelerator/dp/B005S2BKDQ