If the chasis code matches the body then it's all good,
the fibertop code should be starting with BJ74 and they mostly came with 13BT or even the occasional (underpowered for this vehicle) 2L but I dont think they came with a 3B,
however the engine is a minor consideration if installed correctly and in good condition,
your main concern needs to be the chasis code,
if the chasis code doesn't start with BJ74 walk away, a body swap is only legal for the same series.
Check the chasis code from the chasis itself, not the documents or the VIN plate and let us know...