Electric Probs

My electrical setup:

1971 Dodge Demon with stock wiring.

78 Amp Alternator, 3 months old

Had the Mopar orange box elec.ignition system but I put in a GM HEI ignition control module instead between the coil and the distributor, made the car run much better

I have a alpine amp(not big, like 160 WATTS i think) getting power directly from the battery with the on-off switch from the radio.

The problem is this: I've been wanting to put an electric fan instead of the clutch fan. I installed a relay and the fan gets power directly from the battery. The idles fine in Neutral or Park but when I put it in Drive and the RPMs go down the electrical systme feels it. I'll start adding the headlights, turn signal, wipers, etc. and the car will die. What's the deal?

I'm planning on bypassing the AMP gauge as stated at Mad Electrical and putting the headlights on relays as well but will that solve the problem? What do you all think?