Be very very careful.

I had a 1990 F250 with the 460 efi and automatic.

The E4OD , which came in prior years with the 351, wasn't available until 1990 for the 460.  It was (is) a sub-standard transmission.  I made it until 2005 when I sold the truck.  At the time I sold the truck, the trans had a 'shutter'.  Regarding maintaining, it had services at 30k, 60k 90k (but not at 120 cause I was dumping it then) and every trans repair guy I talked to was AMAZED that it had lived so long. ...

Regarding mileage, don't know my gear ratio (or have forgotten it), but best I'd get is 10 or 11 mpg with OD.  Worst would be 7ish towing. 

Bought diesel in 05 and jumped to 9ish towing and mid teens empty.

Bought newer diesel in 18 and jumped to 12/13 towing, and HIGH teens empty. (best empty is 20.2)

edit; the C6 is a reliable trans, since the truck is a two wheel drive, what about adding an overdrive unit behind the trans?  In years past, seems like that wasn't that uncommon of a thing?