they had "heritage" 250's for sure as well. maybe even 350's, but i believe it was only into 97, no 98's.

if you still have this "97 351" could you get a picture, or compare it to your other 351? if it is a mod motor you will be able to tell right off the bat because the are wide as all hell compared to a windsor
modular motor
http://image.musclemustangfastfords....rbo_system.jpg
351w
http://www.riot-horsepower.com.au/351w%20long%20mtr.jpg