I have a Texas Speed LS3 in my '69 Firebird track/street car. The car is nearing the end of the build phase...so it sits a lot. I will place a Battery Tender on the battery every once in a while to charge it up. I have an Optima red top battery mounted in the trunk. The motor is wired with a Speartech harness provided with the motor. I recently tried to start the car...even after placing it on the tender for a couple of days and having the green light on the tender showing. No start. I then jumped it with another car and it fired up. After running for a while. my IQ3 digital dash voltage gauge was reading just over 12 volts, even if I revved the motor. Should I be looking at the battery, alternator, wiring harness, etc? Thanks!

 

Sounds like alternator. When running, it should read at least 13 volts

 

Mine starts out at 14+ volts and settles down to around 13.8V on my gauge.