The Astron 4G54 engine has origins dating about ten years earlier than the 4G63 series. It's a very basic design and built too heavy, though it needs to be to cope with the harmonics of a large capacity 4-cyl engine. It is physically longer thanks to its wider bore spacing and timing chain components up front. It has been mounted transverse before in vehicles like the Chrysler K cars and the Magna in Australia, in fact the first generation Magna which is basically a 5th gen Galant (same chassis as the 6G Galant) had to be widened 2.5" in order to fit the 4G54 transversely, so there's no hope it will fit into a 6G Galant.
If I were ever to do an engine transplant to the 6G Galant, it would probably be a 6G7x series V6 as we know they already fit in the engine bay (in a 5th gen Galant).