I'm considering replacing old Jensen speakers in my 67 Fender Twin Reverb with a single 15" JBL D130F.  The problem is that the current speakers are 8 ohms wired in parallel to match the amps 4 ohm output while the JBL is 8 ohm.  Any problems or potential damage?

Fender transformers of that era will handle a 100% mismatch without trouble. You will lose a tiny amount of power and hear a bit more compression, but nothing bad.
 
If you want it perfect, simply remove 2 power tubes and the amp will want to see an 8 ohm load. You will still have a loud amp but it will run cooler at about 45 watts.
 
The hardest part is changing the baffle board. Some of those amps have glued in baffle boards and some are screwed in from the back. If you make a new board don't use 3/4" plywood or particle board. 1/2" ply is more than enough and it breathes better.

Thanks!!!  I had read somewhere that removing the 2 innermost tubes would do this.  Still plenty loud for my applications

Yes, the two inner tubes or the two outer tubes, not two on the left or right. It should sound amazing with that JBL. Enjoy your mod and do as little damage as possible to the original amp.

another option would be:
 
purchase an extension speaker cab, with the dimensions/speaker/ohm rating you want, remove the speakers from the combo just to make it easier to carry around, and that way you do not alter the original at all, if you ever want to sell it.
 

http://www.jt30.com/usenet-articles/speakers/d130f-history/
 
a little history.
 

I did that with a Twin that I had (AB768 chassis).  I ordered a 4 ohm JBL K-130.  Improved the Twin for the Pedal Steel Guitar. 
 
There are 4 ohm speakers out there.  The Eminence EPS-15C was designed to be a direct replacement for the Peavey 1501-4 Black Windows but lighter.  The EPS-15C is a neodymium magnet speaker and only 7 lbs.  Lightens up the heavy Twin amp.  The EPS-15C will work for guitar or steel equally.   But, if you want a Black Widow, the 1501-4 is a good alternative.
 
If you put in a 15" speaker you will have to move it to one side, not in the middle of the speaker baffle so it will clear the transformers.
 
The Peavey 1501-4 was originally designed to be a higher power handling speaker equivalent to a JBL.  When I first got the Eminence EPS-15C and was testing it the first thing that I thought of "this sounds just like the JBL in my Twin Reverb".

spacealf
http://www.jt30.com/usenet-articles/speakers/d130f-history/
 
a little history.

 
 
D130f enthusiasts have had the opportunity to correspond with Mr. Gerst over at gearlsutz.
 
My D130 cabinet is probably my favorite cabinet so I thought it was great to read his comments about the series.

I have these I think (guessing without looking - they sit around still to be used).
I think I had to have the 10" re-coned, probably bad amp or something.
 

http://cn.jblpro.com/pub/obsolete/k_series.pdf

http://cn.jblpro.com/pages/pub/components/2402.pdf

http://cn.jblpro.com/products/vintage/
 
So they say the D series handles half the power of the K series, so that would make the D130F I guess handle 60 watts, sine-wave power, while continuous music power is twice that I think if remembering right, or about 125 watts.
Well, it is all there written out, if anyone wants to read it. They do not list the D series, and although for a wide-range music system I got the bullet ring radiators, 10" and a 15" K140, it would probably have been better to get the K-145, but still with the self-made cabinets (not all that great at all), and each in its own cabinet, it is heavy.
 
Stick a speaker in an open-back cabinet and I think the power rating is cut in half again, so open back to me for the D series (F perhaps) would be 30 watts then about (35 watts) while providing a back (making the speaker cone stiffer in the process) would make it handle more wattage or the 60 watts, but that discussion is on the history link I suppose, so depends on what you put through it. In my case with the 10" speakers, I was putting a synth through it and both through a 100 watt peavey amp (still kind of works I guess maybe).
 

I subscribe to the theory that a speaker that is being destroyed by too much power sounds like it is being destroyed.
 
You get PLENTY of warning if you are hurting the speaker, and so I've learned to not worry very much about power ratings on speakers as long as I'm the guy making the sound come out of them.
 
For example; I frequently play a Ampeg SVT through a circa 1960 D130 and I have never been concerned that it was being damaged. My personal threshold for damage is well below what that combo will do to you before it gives up.