1988 Crown Victoria: Slipping in Overdrive

I have a 1988 Crown Victoria with a 5.0 engine--with 110,000 miles on
it.

A few days ago when I stopped the car wouldn't go into reverse. After
a while of shifting through gears, shutting the car off, restarting
reverse suddenly started working again. I drove it 20 miles to my
mechanic. He found that a bracket ( or something?) holding a cable
just under the fuel injector had broken and got another one from the
Ford dealer.

Currently the transmission slips in overdrive. If I put the
transmission lever into Drive instead of Overdrive the transmission
works OK.

Any thoughts of what could be wrong and possible fixes? Is it fixable
without dismantling the transmission?

What are the consequences of just driving with the car in Drive? How
likely is it transmission will go completely out soon? Would it be
safe to take the car on long trips? Any problems driving at 75 miles
per hour? How much will it reduce high fuel economy?

What's your experience with junkyard transmission replacements for
this old a vehicle?

Thanks.