We have a 1954 Ford F100 that has what is believed to be a 256.We have a new 360 will it fit?

It will fit as the engine bay in them beasts are large enough but if you are asking, is it a direct bolt in the answer would be "NOPE". You would have to swap engine and tranny as a unit, do a lot of rewiring to accommodate the 360 and any sensors and the ECU if it is not at least a 30 year old engine and also lots fabrication like motor mounts, perhaps having your driveshaft shortened, etc. Also them old trucks have a real low geared rearend so if you plan on turning it into a cruiser you need to change the rearend to something a little more engineered for today's highway speeds as well as upgrade the brakes. It's a whole lot of fun to go faster then hell and a whole lot of pain if you can't stop that freight train when you need to.