Thanks to Sheriff Buford T. Justice:
Ford recommends ATF change at 30k for normal use for 96-97 SHO and at 60k for 98-99 SHOs. Based on my analysis of used AFT fluid I would get the system flushed every 30k to be on the safe side. By 50-60k the transmission fluid is spent. I changed to Mobil1 AFT at 30k and the transmission still failed at 45k.
In my case, the vane pump in the valve body may have bad metallurgy and with diminishing fluid flow I got problems with false neutrals and lost 3rd and OD.
Maybe because I am an engineer - or maybe because I believe in redundantly paranoid backup systems I felt it prudent to check out aux transmission coolers and put on on during the rebuild.
After a lot of reading I chose B&M 19,000 GVW for several reasons.
Cost which includes mounting hardware was $46 at Summit Racing or Jegs. First the ATF goes in & out of factory radiator then through the B&M. (series) I was told if they ran in parallel the fluid would take the "easy" route and one cooler would not get used.
Using the supplied kit you use simple wire tie type thingies to mount the B&M cooler in front of the factory cooler. It is very simple.
So far all has gone well, no leaks. I don't have a ATF temp gage so I can only assume it is happy in it's work down there.
Instructions for installing B&M cooler in PDF Format. ( right click & save target, it is a small 60k file)