Just finished my ls swap and when I go to turn the key the starter only clicks once then nothing. Switched out the starters and still the same single click. Any ideas?

 

Weak/old battery?

 

Well we have a booster box connected to it while trying to startup, lights in the car don't dim as if the battery is dead/low power.

 

It sounds like the solenoid is switching, but that there isn't enough power to turn the starter (or there isn't any power at all) so you should check at and around the starter, starter solenoid. Many cars use a small relay to switch the (much larger) starter solenoid on, so that may be the click you're hearing.
I'm guessing you either have a corroded connection (probably a corroded ground connection) or a loose power or ground connection.

If the lights are not dimming, that probably means that power isn't reaching where it should be. If there isn't a big current load then the lights won't dim, if that makes any sense. So if the starter isn't completely connected, the lights aren't going to dim because the starter isn't drawing current.

 

yes, check the ground wires.... also, manually crank over the motor from the pulley. Make sure that the motor is not seized.

 

If it was seized the lights should dim really bad, the starter will draw a shitload of current if the starter motor is stalled like that.