1997 GT engine replacement

Hello,
I'm new to the forum, and I'm joining to ask for advice on a new project I'm about to start. I just bought a 1997 Mustang GT 5 speed with a bad engine, and I'm considering my options to replace the engine cheaply. I'm planning on a junkyard engine, and the junkyard near me tends to have a lot of Explorers. Right now I'm thinking that my best option is probably either an aluminum Explorer engine, or else some other PI engine.
Will any PI engine work with my stock ECU? What years did the Crown Vic, Grand Marquis, and Town Car switch to the PI engine?
Someone told me that the 2004+ Crown Victoria intake would be an upgrade for some reason, is that true? Would it work with the stock ECU?

Thanks!

Thanks for the advice! I didn't know about the plug thread change. Did the Mustang also get better plug threads in 2003?

Like I said I'm also thinking about using an Explorer engine, because there are a bunch here and I'd save 90 pounds. I read that this will require an Expedition front cover.