I have a 1990 f-250 with a 460, but hate the gas mileage. Can I downsize it to something like a 351 without changing the tranny(auto)?

Yes you can.

Motor mounts may be different, but no biggy.

Wiring harness probably is going to be different, hope you have a donner truck for parts you are going to need.

So you want to spend hundreds- of dollars- and work hours- because you want to save a few dollars on the already cheap gas? why not just buy a truck that gets better mileage?! just askin'

Do the math....lets say you presently get 12 mpg (@$2.00) = .166 cents per mile.... If you improved that to 16 mpg (@$2.00) = .125 cents per mile. That is .041 per mile savings, that is a saving of $41 per 1000 miles....or $410 per 10,000 Use those number against what it will cost you to find and install a different engine. How many of you remember back around 1973 when gas had been .30 cents per gallon and the price doubled and gas stations were closed.....people panicked ....traded in nice big cars for economy cars and took huge lossed on the their trade in because they thought the gas world was coming to an end.

For even better mileage convert it to a 5.4 Triton engine and 4 speed transmission from a newer F250. Get the computer and the wiring harness or even better yet get the donor truck complete.