Times & prices have changed. It seems that the cheap SP forged pistons are history, unless you can find a set still on a shelf somewhere.
From what I can find, in the last few minutes, good forged 389 pistons will run about $600 + tax.
DSS has the 389 covered. They make 'em for early or late type heads. Also offer 4032 or 2618 alum.
https://dssracing.com/Pontiac-piston...55-stroker.htm
IF your block will go to 4.125, & you wanna take it that far, you MIGHT be able to find some early Olds 455 std bore pistons. This has been discussed here before. These dish pistons should work with early or late heads.
SP forged are # L2323F std. Here's a set + moly rings, for $385 + tax.
https://www.ebay.com/itm/17105739891...gAAOSwr41XH4BP
The SP number for 400 Pontiac std bore forged piston is L2262F std. But good luck finding a set. It appears that SP quit making the 400 & 455 Pontiac forged pistons. Summit currently shows that they can special order a set for you. But, I suppose that would depend on whether or not SP actually has a set in stock at one of their warehouses. Price: $456 + tax
https://www.summitracing.com/parts/slp-l2262f
This place shows to have a set + moly rings in stock, for $410 + tax & shipping.
http://store.enginepartscenter.com/p...0784-p933.aspx