You're looking at that cusp between pushing the stock block and it not really being worth building a forged one IMO. I do not feel the stock block is good to 350whp, I rarely see that work out for long. You can do 320whp on a 16G very reliably. 340whp is really asking to lift a head or crack a ringland if you get any knock at all, so I'd say that's only realistic if you run on E85 and are far far away from the knock threshold. I've seen people get away it with for quite a long time, but usually it bites them in the ass sooner rather than later. Search '20G' and 'YNANSB'.
I honestly feel anyone who puts the stock head bolts back in a motor that will run more than stock boost deserves a swift kick in the nuts for being a short-sightsighted cheapskate. ARP studs should be considered mandatory any time the heads come off unless pulling the motor is your favorite hobby.
A new shortblock is a really good bang for buck move. If you're okay at 300~320whp I'd keep the stock pistons but open up the top ring gap a little. Add some head studs for reliability and have your fun. If you want 340~350 you can get away with the Mahle 4032 pistons if you're that concerned about piston slap. They're expensive but high quality. One big drawback here is they don't do a piston for the stock bore size, you have to bore the cylinders. Another is that come the day you decide that 340whp isn't doing it for you and want to go bigger, you will always know the pistons are a weak link. This is the opposite of future-proofing.
For more than 350whp you probably want to use 2618 slugs. Do not be tempted to run 2618 pistons at tighter clearances than the manufacturer's suggestions, I've seen this go wrong so many times. Upside here is that Manley does offer a graded, stock-bore piston for us. Check the ring gaps and drop them in.
TBH I think all this talk of piston slap and short life on 2618 pistons is somewhat overstating the problem. Yes you will get a bit of clip-clopping in winter until the motor is fully warmed up, including at idle if you were just cruising. Yes you'll get some in summer when idling with the a/c on. I don't have any oil consumption to speak of after 45,000km on my built motor, actually don't use a drop except during turbo or feed pipe failures. It's not any louder today than the first time it fired up.
I'd say people have unreasonable fears of forged pistons for the most part but there are some contraindications. You do not want a forged motor if:
You do a lot of short trips where the motor doesn't come fully up to temp.
You insist on your right to just stomp on the loud pedal before temps are up.
You insist on using thin oil like 0W-whatever or 5W-whatever because it saves gas.
You insist on using cheap dino oil or non fully-synthetic oil.
You think 5,000 miles between changes is fine and saves you money.
You have a golden ear and cringe at every mechanical sound a vehicle makes.
Otherwise, don't be so timid.
Oh, and read this: http://legacygt.com/forums/showthrea...st-178684.html