I'm in the final stages of interior reassembly (yes, I'm painfully broke!) What are my speaker options for the 68? I think it came with 4" speakers, but it looks like a 4X6 will fit. Stamped on the radio (original), it says use 8-12 ohm speakers and I'm told a lower ohm speaker will damage the amp.
I'd like to get a decent sound with the old system, but obviously not expecting much.
Any thoughts?

My one area where I feel like I can add some value to this forum... I wouldn't go below 8 ohms, as it can cause your radio to overheat AND will not produce optimum power at the speakers. Bigger issue is that you need all the volume you can get in a convertible or T-top car, even more if you have loud exhaust. In order to produce anywhere near maximum efficiency for volume and bass, the kind of speakers you use in cars need to be installed in a sealed compartment (behind the speaker). I installed a modern hi powered stereo and 6x9 speakers in the back of my '81, in the factory locations. What a disapointment. I could barely get enough volume to hear the radio while driving. I came up with the following idea, which produces GREAT volume and bass:

Removed the rear compartment doors and trim ring.
Removed the doors from the trim ring by removing the wire "hinge".
Cut 3/4" plywood to the shape of the doors.
Covered the new wooden doors in black carpet to match my interior.
Installed the 6x9 speakers in the new wood doors.
Wired them in with enough extra wire that I can remove the doors and get tthem out of the way for battery and jack access.
The result is it sounds great!
1 more tip: I cut the new doors to just the right size so that with the carpet, they fit snugly in the trim ring without anything else holding them in. Works great...

Sounds like a pretty unique option!

My 76 has a kenwwoood stereo,cd and new 4" speakers in the front in the original dash speaker area and 6 x 9 in the back each in a custom box built by a local Custom Installed car stereo company here. It sounds great! the boxes are against the flat wall of the storage area behind the seats. My vette only had front speakers originally, which were not very good speakers. The owner before had it installed. I love the sound!