Had these delivered this morning they lot bigger then expected was going put them in the case to push smoke back to the vent now thinking to turn them into extractor fan 1 powers out at 2300 rpm thinking both together and build case at back would they be better then the fan I have now
I’ve just got my extractor too. I’ve gone for an in-line centrifugal bathroom extractor with flexible hose. it will be interesting to see the difference between yours and mine. Please keep us posted.
Two fans mounted on top of each other will give less static pressure than having them side-by-side. But the air flow may increase. Test it 
I was going to do side by side
Unless you’ve also opened up the intake side of things (get more air into the box) all the exhaust mods after getting rid of the rectangular exhaust manifold will be of limited value. The stock K40 usually has a single 2" hole in the bottom for air intake to replace what you’re sucking out of a 4" square hole. Open the lid an inch or two while using the stock exhaust and see how much better it is.
Ditto on @Jim_Hatch 's recommendation. Cracking the lid made a huge difference for me.