when it rains it pours

Instrument voltage regulator is inside the 3 post rallye fuel gauge. You've ordered a part that you car cant use. Maybe you can send it back. The one built in your gauge may not be damaged although it does get its power from the same blue wire.
The ALT gauge... It has a red wire and a black wire. Make a mental note of where red is if you intend to reconnect those wires. RED is written on the printed circuit board but you wont be able to see it during install. If you would rather just hook those 2 wires together and have a dead ALT' gauge, go for it.
No real need to "cut" into the harness. The blue wire goes to the bulkhead connection so you will be going there too. So I suggest you go under the hood and disconnect those 3 plugs. Push the bulkhead connector inside the cabin. Go inside a bring the bulkhead connector out toward you for service. Next you can unwrap what looks like electric tape, replace a wire and wrap the harness right back the way it was. A bunch of gluey electric tape is just a mess next time.