FWIW……
Here is a quick look at what Summit Racing has for pistons for the 318 & 340.
318’s slugs below.
Hyper-U’s
KB-167, flat top, 5cc relief … $391
KB-399, dome, +6cc …………. $348
Forged
Icon IC844 flat top, 4.70cc @ $659
Icon IC845 dome @ 4.90cc @ $659
340 slugs below
KB243 (The only hyper-U) flat top 5cc @ $323
(All forged below)
Speed Pro L-2316F (forged) flat top 7.5cc $366
Ross 99626 dome @ 12.60 cc $638
Ross 99625 flat top 5.5cc $640