A "granny" gear is an extra-low speed 1st gear, and it's probably the NP-435 4-speed. But since 1st is only used for towing & climbing, you'll only use 3 of the gears for normal driving. It's what my truck has and it's considered to be one of the strongest manual transmissions ever built. Even 2 1/2-ton trucks used it. There were several other 4sp transmissions used around '87.
If it's a 5-speed, then it has to be the ZF which will have "ZF" cast into both sides of the transmission. The only other 5-speed it could have would be the M5OD, which DOESN'T have a granny 1st.
Here's what they all look like:
The sticker in the door jamb will tell you what the GVWR is, but you need the GCWR to know how much trailer you can tow. That may be harder to find for such an old truck if the owner's manual is missing, but it'll be similar to new 3/4-tons which you can find here:
I frequently pull 5Kip with my truck, which only has mild mods to the suspension, and I've towed MUCH more on occasion; somewhere in the 12Kip range and once even a 13-yard Peterbilt dump truck (empty).
The truck you have is a lightduty Ford its got the 6600 gvw you gotta keep in mind these trucks are no more than a beefed up 1/2 ton because they got light springs and a semi floating axle. The transmission in your truck has gotta be a T-18 4spd they haven't used the NP435 in Ford trucks since the late 70s I would rather have a T-18 they are a nicer transmission to shift.
As for what you want to tow make sure the brakes are good in the truck and you got good brakes on the trailer and a good brake controller for the trailer. You also want a heavyduty receiver hitch get one with a 10,000lb rating without equalizer bars the heavier the hitch the better. The truck your looking at will be maxed out at 6000 pounds depending on where you live if your in hilly areas or will be pulling in mountanous areas don't exceed 6000 lbs.
The 302 engine is good but remember its a small engine don't expect it tobe a speed demon take it easy your not gonna be winning races pulling a trailer.
The F250 is by no means a bad or incapable truck, but if I was buying a truck specifically for towing a heavy trailer, I would be looking for a one ton dually with a diesel or a big block v8. If you get the regular cab 2wd with 4.10 axle gears, you will have the highest possible weight rating and trailer towing ability in a pickup truck. With the older trucks in the price range you're looking at, there isnt much difference in price between the duallies and the other trucks, may as well get as much truck as you can...
You don't need a 1-ton to tow 6Kip - I tow that with my old Bronco all the time. It has trouble when I get over that, but I have a straight 6 and the shortest wheelbase.
My truck weighs 6Kip with all the mods & cargo I normally carry - factory curb weight for Broncos was only about 5200. A 16' trailer weighs about 700lb and a car shouldn't weigh much more than 4Kip, so you'll only be towing about 5Kip or less.