Coolant temp sensor: Do you mean that the range for the Howell and Speedhut senders are identical (not just the plug/shape/threads)? My only worry when you start splicing into these kinds of wires is that fractions of an ohm can mean a different reading.
Somewhat similar on the fuel sender: they always seem temperamental, especially related to grounds. From what you're saying, can't you just use the Painless sender wire and at least pick up the ground where the Painless sender ground hits the chassis? Do you have to drop the tank to get to the Painless sender ground? I don't see any problem reusing the signal wire. You can certainly try to just ground the fuel guage somewhere near the dash and see if it works though

Worst case, you run back to the Painless ground, right?