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Look at the warranty plate on the driver's door under the striker. GVW is 4500-5000 depending on springs. And 302's came with C-4's. C-6's were used with FE's.
There was a small block C6, but not sure available in pickups then?
BTW, gorgeous truck!!!
I used to haul an open trailer with my '64 Galaxie wagon on it behind my 1970 F100, longbed, 302 w/toploader 4spd(was 3 spd truck). Did just fine. Mine did have the Flex-o-matic springs on the back.
The FE 390 is a different engine family so it couldn't be swapped over from the 390. Double check the number of pan bolts and shape to differentiate between a C-4 and C-6.
Have your truck weighed in the meantime too, so you know what your exact capacity is for legal reasons.
The GVWR (gross vehicle weight rating) is the maximum weight, including cargo AND the weight of the vehicle. So if yours is for example 5,000 lbs, and your truck weighs in at 4,000 lbs (with you and gas in it), you officially have a 1,000 lbs capacity between the bed and the trailer hitch.
For the hitch it's just tongue weight though, not the whole weight of the towed package (if any) which are separate. Load capacity is separate from towing capacity.
That's the way I understand it anyway.
My F350 4wd for example has an 8550lb rated GVWR and weighs in at a hefty 6,500lbs. Making it literally just barely a "one ton" truck!
So while it will carry a LOT more than it's rated capacity, I can only carry 2,050lbs (of passengers, gas and cargo) if somebody checks and I get weighed.
Even though I was loaded to the max one time for a trip through Washington state, the suspension hardly settled down in the rear.