question,
I have a 289-.60 over/4 speed toploader. I wanna drive freeway and tow my camping trailer (only a single axle 16 footer). What's a good ratio for the 9" I will be putting in?

 

I'd probably look for a 3.50 with your combo and planned use. With the 4spd tranny you'll run about 2500 rpm at 65mph. Low gear in the tranny is probably adequate for takeoff with the trailer - especially if you can start out in second gear running empty.

I run a similar combo in my 86 150 so the above is from experience. Only variations would be our first gear ratios and tire size. I'm running 235 75 15 LT's.

 

I see camping trailers being towed by ricers so they must be pretty light.
As long as the truck bed isnt full with beer you should be OK with 3.50's but expect to be shifting more often. The 4spd should make that easier to maintain the torque rpm range.

 

3.50's would probably be your best bet given your description. You MAY also get by with 3.23's (???)

Daryl