concours USA-6 custom auto sound radio.

You should send this question over to the Audio section of this site. There's a lot of good advice over there.

But, with that being said, I doubt that without an amplifier you'll get over those mufflers.

not enough power, and then with more power, those speakers won't be enough

I just installed this one in my 68 and it sounds good.  I added a set of 6 1/2 in the kick panels and have 6X9 in the rear deck.  The nice thing on this is two sets of RCAs in case you want to add a sub later.  I decided to go ahead and run an amp in the trunk with 2 10' subs.  Might split them to individual amps later with second set of RCAs.
http://vintageautoradio.com/shop/index.php?main_page=product_info&cPath=8&products_id=54&zenid=e5720f7369d317fca5920ffbacd7f1c8
 
Reasonable price, other models have 1 RCA and cassette deck or support ipod input.