Are all engine blocks and heads the same

Hi, Well i have started looking for an engine, a couple of engines are on e-bay, what i would like to know is what options do i have, if i choose a 3.9 serpentine engine will i have a problem with the crankshaft sensor or can this be fooled to send the signals required. If a buy the THOR engine on e-bay do i just strip everything and replace with my 4.0 components. And if i buy a 4.6 would have to just alter ecu so it runs ok or try and get the original with the engine.

dont buy one off ebay as there is a good chance you buying ****, try and get the same as you taking out..

If you want to fit it in yer P38 it needs to be a 4.0 or 4.6 that ran GEMS or THOR injection otherwise you wont have anywhere to stick yer crank sensor.