Where does the sender go? There is a spare plug in the intake, opposite the temp sender. Will this work?

Stock Bravo cooling system

Thanks, the block drain will work fine.

Next question.... Can I put an normally closed pressure switch in the other side to turn on the alarm/buzzer? That way if I lose water pressure for some reason it will alert me before it gets hot.

You started off right use the intake manifold cross over not the block drains they will collect crude and rust that will plug it up. The switch idea is not bad but you should see the pressure on the gauge at low speed, at higher speeds the drive or transom pickup usually supplies more water then the system can use. Many guys that that do the higher speed have trouble with to much pressure. If a pump fails at speed you may not know it until you slow down, I can not tell you how many times I had a customers come in saying I ran X miles no prob then it over heated just as they came in.

Phil