will a lpg tank out of a el sedan fit in a ba sedan

No mate, Nowhere near..
Ba runs a MUCH smaller tank than E series.
E series use a 86L tank (68 usable) and BA runs a 86 (approx)l tank with 63 L usable.
Mountings are completely different too. BA's have brackets welded to sides of tank and bolt through wheel arches.
E series have legs welded to tank that bolt to boot floor.