V8 EFI ECU interchangeability

Hi all,
I have a series 2a with a 3.5 efi v8 engine which i has very low oil pressure. (oil light on on tickover)
I have bought a good 3.9 serpentine engine with distributer to swap it out,but did not come with a ECU. Can anybody tell me the correct (best) ecu to attach to the engine.
There are so many different ecu module numbers on ebay for this engine. Im sure the afm is a hot wire.
Any help would be appreciated.

Just found a picture, my 3.5 efi is a flapper type, not hot wire as stated above.

You'd be best getting a used hot wire system, I have read somewhere it's hard getting the flappers to run just right with a 3.9 apparently they lean out too much further up the revs unfortunately. The bonus is 3.9 injectors are peanuts compared to the 3.5s, I think a hot wire system would be preferable in this case even though the flappers are great, you could try and adjust the flapper spring and bypass screw and see if you get the correct mixture all the way from idle then up the rev range.