4.6 would have to be a very tight fit .A 390 or 428 is tight ,hard to get the center two plugs out. i think the plugs are on top on a 4.6 though. i have never seen a 4.6 in a 69 or 70, if it would fit with no tower mods that would be nice . doing the 429 Boss tower mods would not be very difficult but you will need custom motor mounts. and i am not sure what you would have to do to the trany tunnel so the trany would clear .
I know a 4.6 will fit in a 71-73 easily, the engine compartment is quit a bit bigger. I have seen this done and it works out great .