1965 models used the radiator fittings and rubber hose. You can get the kit that has the rubber hose pieces and the correct fittings for the radiator. Instead of getting 65 cooling lines, you could probably just cut the ones that you have now.

 

Only to my knowledge did the 65's use rubber hose to connect the trans cooling lines at the radiator. Since the tube nuts on flared lines can, will, and do rust fast to the lines many have been "fixed"by adding rubber hose. I just finished installing a radiator in my 66, and connecting the cooling lines. The radiator had 1/8 NPT (national pipe thread) female openings at the bottom. My transmission cooling lines (new stainless steel) had 5/16 male inverted flare ends. I used a brass fitting adapter 1/8 NPT male to 5/16 female inverted flare. I used teflon tape on the 1/8 threads into the radiator, be careful if you use the tape, do not start the tape at the beginning of the threads. Any small piece that may come loose could cause internal transmission problems.

Should you need or require a photo, let me know I can take one and email it to you.

 

Only to my knowledge did the 65's use rubber hose to connect the trans cooling lines at the radiator.

LOL, maybe I only looked under 65 mustang bonnets.

I used a brass fitting adapter 1/8 NPT male to 5/16 female inverted flare. I used teflon tape on the 1/8 threads into the radiator, be careful if you use the tape, do not start the tape at the beginning of the threads. Any small piece that may come loose could cause internal transmission problems.

I think I have the same fitting. Male to radiator, female to cooling line. I suppose they're a common application or are there other sizes possible?

Maybe I'll disconnect them again and retape them. I didn't let the tape go past the beginning of the threads. I put it in and turned it to the point that it fitted snugly. I didn't want to turn it any further, afraid to break anything. Oh, I also used tape on the flare nut on the cooling line. That doens't hurt does it?

 

There should be no tape on the flared nuts. The seal is made inside between the cone and the seat. Tape can actually cause the fittings to leak as they can prevent the nuts from fully seating. Remove the tape and try again. Make sure to hold the brass adapter fitting with a wrench as you tighten the flared nut, that way you can make sure it is tight enough.