Yah, 10-4, I saw that later.
Assuming the top of your radiator is below your thermostat housing and fill point, the goal is for your radiator cap to never open. Note this advice to someone else in another thread.
Given you are not going to swap the radiator out, you want the radiator cap set as high as possible, you do not want it to open. Hence the reason John is recommending a 20# cap on the radiator vs. a lower pressure cap on the thermostat housing.
No. For starters, drill a hole in the cap you presently have. At some point figure out a better setup that vents down do it doesn't get rainwater or insect nests in it.
I'm still building my LS boat, I have not gotten that far. On my swamp buggy I run my hose to the expansion tank through a hole I wholered out in the lid of an empty anti-freeze jug, made sure the hose runs to the bottom of the jug and tie wrapped it off. That's a vented expansion tank.
The point here is that you want one single place in the system that vents off excess pressure and you want that to be at the highest point of your cooling system. When anti-freeze gets hot, it expands, some of it needs to go into the expansion tank. This also vents off any air in the system. When the system gets cool, it sucks that coolant back into the system from the expansion tank. The expansion tank should only be 1/4 full when cold and the level when cold should remain stable, otherwise you have a leak.