Looks like a good pick up over all, nice one
The Sigma/Scorpion mounts that should come with the engine will be fine.
Yes, you will need to get the tail shaft modded. Best way is to grab the Scorpion tail shaft and have the rear yoke changed to suit the Galant diff, and shortened if needed.
The alternator mounts on the other side of the engine on an Astron so you will need to run the dash light wire around the engine bay to the alt and mod the main alt wire to suit.
You will need to remove the spacer between the radiator and the rad support panel. This will move the rad about 2 inches further away from the engine, as the Astron is longer the fan will foul on the rad if this isn't done.
You will also need the speedo cable out of the Scorpion, the connections for the dash and gearbox are the same but they are longer and the speedo drive on the gear box sits further back on the 5 speeds.
While talking about the gearbox, you will need the "short" extension housing. Basically, you need the gear stick to sit forward of the rear seal, if the gear stick sits over the tail shaft yoke it will sit too far back on a Galant and foul the hand brake. On the gear stick, you will need to trim about 50mm from the rear of the factory hole, you can spin this around and weld it to the front of the hole, then the size is the same and you can use the factory floor boot.
I think that is about it. I know the factory automatic bodies have a larger trans tunnel, if yours if factory manual I am not sure how tight the 5 speed will sit in the tunnel.
And the obvious exhaust adjustments.
Extractors to suit a Scorpion/Sigma will not fit as they will foul the steering box but some minor mods can see them used. Having said that, the Scorpions ran a very efficient "extractor" style manifold that will pass the steering box.
This may seem like a lot reading it but as you are starting with a rolling shell, besides the tail shaft getting modded you can usually complete a conversion in a good day or a relaxed weekend.
Good luck
