The "Boss" design was brought on scene in the 60's to rival the "Hemi" head of the Chrysler corporation. Small block boss heads tried to emulate a hemispherical design without truly being 100% hemispherical. Not a bad job though. Combustion chambers were opened up to allow good flame propagation and the spark plug location benefited from this because of the "Cleveland" heads this originated from. 302 engines always had a "Windsor" head until the introduction the 4 bbl cleveland head with the afore mentioned "Boss" design. They got the "Boss" designation on these specific blocks. 351c engines had the cleveland heads and depending on whether they were 2bbl or 4bbl production they could get and exchange the Cleveland head style. The Cleveland head had canted valves which means they had staggered valve placement compared to inline placement. The standard inline placement of a Windsor head approximiately 23degrees in relation to the piston. The staggered Cleveland head was produced to place the valves over the piston for better flame propagation and air flow across the cylinders top during piston quench. The 4bbl Cleveland and Boss heads had very large runners and the Boss enjoyed pushrod guideplates. The 429 boss engine was truly a "Hemi" and the originals offered a "T" designation or a "S" designation and came with a "Big" Holley to feed the beast. This is a short and abridged synapsis of the storied history of the "Boss" and I know there are better experts with good detail on these things to fill in the blanks. Are you shopping specifically for a boss or at least toying with the idea of running one? Would be fun to run.