If you haven't yet do so, revisit the linkage setup. Have your tried going into first then reverse. They are on the same shaft. This is, especially, helpful when idling to prevent clashing?

 

Is the cable for the reverse lockout broken? (Asuming it is a 4speed toploader.) Is this something that just happened? Welcome to the forum.

 

No i just brought home on the trailer from the restoration shop and it wouldn't go into reverse to back off the trailer. A friend crawled under and said it was a linkage issue, that it needed to be aligned. If this is the case, I 'm not sure what that process is. Thanks

 

 

It is a hurst competition shifter, I should have included this in my original post

 

Put the shifter in N, have your buddy hold the clutch pedal down and go underneath with a 1/2" and a 9/16" wrench and see if you can move the R shift lever on the side of the case by turning the nut that holds the shift lever onto the transmission.

Surely you can find the adjustment procedure for the Comp Plus shifter on Hurst's website.

 

You next purchase needs to be a a Ford shop manual for the car From NPD. Not a Chilton or other manual.
SHOP MANUAL REPRINT OF ORIGINAL 1967 MUSTANG - #L-25 - National Parts Depot

Be sure to request a catalog. It’s invaluable

 

Look at at the side of the shifter. there is a quarter inch whole that goes thru the shifter mechanism. Unhook all the rods from both ends( shifter and tranny) . Slide a 1/4"drill bit through the hole in the shifter, this will lock it in neutral position. Make sure all the arms on the tranny are in the neutral position and adjust the rods to fit without moving any other linkage. It won't go in reverse until everything else is in line. just did mine. piece of cake!.