LS3 with a big cam is a 500 crank hp engine. You're losing displacement at least. Not ultra certain on the chamber size difference between the two head types. It will run quite well if you just slap it together and ignore the numbers game.
Where is the best place to get pistons? is there any one in particular manufacturer that everyone likes? I am looking for a set of pistions with about a -4.00 Piston volume but maintaining the stoke bore.
Get the whole thing, harness ECU and pedal. Swap as a unit. Absolutely the easiest way and the cheapest. Lots of cam swap videos online, it’s pretty easy to do in an LS. Either that or do tons of homework and hunt for a year or two for deals on Craigslist or Kijiji and piece it together.
Personally I wouldn't get hung up on a hp number... Get an engine, toss a cam in it and the accessory drive/intake system you want and get it in and running. I know it's sounds stupid to then have to possibly pull it apart later to get more hp, but converting it is a big hurdle and to be able to drive it afterwards instead of languishing waiting for money to buy more swap parts you forgot sucks... Use a 5.3 or 6.0, toss the Elgin cam and some springs in it, and you'll be over 400hp easy with headers.
I am planning on using a LQ4. I have time to save money so thats not a big concern at this moment. My next concern is if i replace the pistons will I need to rebalance th crank. I will most likely use these https://www.summitracing.com/parts/wis-pts523as to lower the compression. if I use stock rods wil I need to rebalance the crank?
Balancing is an important part of any build as far as I'm concerned. It needs to be done if there is a change in bobweight (sum of parts hanging off the crank throw- piston, pin, locks, rings, rod, and bearing). If those pistons weigh exactly the same as stock, you're fine. BUT, why would you lower the compression?
Any specific reason why? Not a lot of power to be made for the cost involved, much easier to swap a cam and have some headwork done to make power. Yes you should re-balance the lower assembly after pistons, can you get away with not doing it? The internet will say yes/no/maybe.
What are you trying to accomplish here? Your HP/TQ goals with about 15% drivetrain loss say you need to make around 468hp and 526tq at the crank. That kind of torque would be easier with a 454 or 455 big block than a naturally aspirated LS.