what is the largest engine that will fit in a 1987 F150

good place to start would be: www.4btswaps.com more torque than you probably need.

Depending on your needs, you may also require another tranny.

if you can find one in a similar truck, youll probly need most of it, but the 460 is probly gonna be your best bet, unless you can build a motor to do what you need, it doesnt have to be an 87, and you probly wont find a 150 with a 460, but as long as you get the same body style truck, youll be set, if you were up for modifications, you could get a 429 as well, but you need a real big block, not a "modified wanna be"

I would say to go with a 460 and be done with it .