It's going to be important to factor in the date of manufacture. If it is pre 76 (and pre ADR 27A) then there's stuff all regulations (relatively) but after that, and the further along you go, the more it will be an issue. If you do an engine swap from a newer engine - then the emissions standards that the motor/donor car was subject to has to be applied, even if the car the modern engine goes into is 40 odd years of age!

Never mind about power increases (of which there usually are none with a pod filter anyway unless there's something drastically wrong with the filter on there before the swap to a pod filter) where it will be a potential issue is emissions. You can't exactly have other stuff being fed into or out of some air filters if teh element is removed and a pod put on instead.

You can also find the airbox size and shape was designed to help smooth out airflow going through an afm, hot wire or otherwise, and the pod upsets it.

If you used an airbox from another vehicle, whilst it might technically lead to NVH or other issues (they tend to be optimised for the specific engine that they are mated to) - but I suspect most engineers wouldn't pursue that angle as hard, and if it has a proper airbox on there, it should be allright