Thank you for that advice makes since because at idle volts is 14.3vIf your lights are dimming its not your alternator. Your circuit is not capable of supplying the power it needs in the moment. You definitely need the big 3 if you don't already have it done. If you drive and your voltage starts dropping over time, then your alternator can't keep up.
As far as the alternator, this may not be everything, but:
Kicker is probably pretty close to that rating so lets stick with 2500w
2500w / (roughly guessing) .85 (for an 85% efficient amp) = 2941 watt amp draw
2941 / 14.4v = 204 amps. Is roughly what your drawing.
Its not drawing that constantly, but it is when your bass is hitting heavy, and then recharges your battery between there. It looks like your on the original carbureted motor, which means no fuel injection, you probably have mechanical fans, you don't have an ecu. You have very little in your vehicle that is drawing current compared to anything made in the last 20 years. Your starter when cranking, and your lights are about it.
Your alternator also has two numbers that matter; amps at idle, and whats the rpm for max output. I would bet you rev higher than anything newer as well, and are usually cruising at max RPM.
If that doesn't plan on changing, you could probably get away with an alt rated as low as 150amps. If you were in a newer vehicle that would need to be about 250 amps. You can buy as big as you want, but that lower draw might help open up options for that older style alternator.
The big 3 is much cheaper, and definitely required. Start there if it's not done, then go for the alt.