You NEED a bigger alternator, 100amps+. The stock one might last a month.
I picked up a "Visisonik 300W" amp on ebay for $23 delivered. Puts out 70Wx1 RMS and sounds great. The same seller had a pair of 12s for $40 shipped or so. An amp install kit is $15
2 or three sheets of plywood, a 1x2", a bunch of woodscrews and a tube of caulk will get you a half-assed box for cheap. Stuff it with leftover house insulation (free) and you're in business. if you want to port it, find the Fs of the speaker and use an online calcualator to find out what it needs, then use PVC pipe for the port. BigLots has carpeting cheap.
We built a system like this for my buddies truck, already had everything but the speakers and amps and he's been offered $200 for it. Might have half that in it.

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66 coupe 5.0/T5, home-brewed power 4 wheel disc setup, 350hp+, looks like it belongs in 1968