If your engine guy has extensive Pontiac knowledge, as you say, then you don't need to buy anything from Butler, unless you just wanna support them because they're the big name in Pontiac performance.
You can source your parts from the cheapest sources, to save some $$$. But, if price is no object, then you can let your engine guy source the parts, so he'll make the extra $$, instead of Butler. If he's makin more off the build, it might encourage him to do a better job. I'd venture a guess that some engine builders resent customers sourcing their parts somewhere else, beating them out of that profit.
Wherever you decide to buy the parts, I'd go with forged pistons. The cheapest are the Speed Pro L2262F, in the oversize you need. The cheapest price is almost always from an Ebay seller. Summit & Jegs will usually match a legitimate price you find.
Right now, $305.61 is the best price I can find for the L2262F30 pistons. But, I assume that Summit & Jegs will match that price, if you prefer to buy from them.
Don't know your budget, so I don't know how many upgrades you'd like. If you have more to spend, lighter pistons are a good investment, as are forged rods. I'd recommend Auto Tec pistons & H-beam rods. The pistons are a little over $500 & the rods are $400, from RPM. Some prefer Eagle rods. They're usually about $65 or so higher.
Besides being lighter, the Auto Tec pistons use thinner rings, and they have floating pins. Also a big plus is the fact that you can order them with any pin height you want. This will allow you to order the pin height you need in order to not require that any extra material be machined off the block decks, in order to achieve zero deck height. Any competent machine shop will square up the decks parallel to the crank. But, with most shelf pistons, more must be machined off, in order to achieve the desirable zero deck height.
