Sorry T4 was a mistake. Everything else is true. What your talking about use to be called a bastard 20G. Mitsubishi doesn't make a td05 20g turbo.
Heres a list of factory turbos:
List
The 20G only comes from Mitsubishi on a td06 turbine with a t3 housing. So if op is asking if a mhi 20g turbo will fit his Galant, the answer is no.
TDO5/6-20G custom job. Usually done using TDO5-14B. uses a TDO6 compressor housing. Better midrange to top end. 650cfm@15PSI of boost. 6cm or 7cm exhaust housing depending on turbo builder.
TDO6-20G custom job. Usually done using TDO6 cartridge, using a TDO6 compressor housing and machining a TDO5 exhaust housing to accept the TDO6 exhaust wheel. Incredible midrange to top end. 650cfm@15PSI of boost. 8cm exhaust housing and it will need an adapter plate.
Mellowvr4 if by "porting" you mean machine a td05 turbine housing to fit a td06 turbine than yes that would work.
In order to use the 20g you have to buy or have made a bastard setup. There a few ways it can be done. Like anything else you power goals dictate the best rout.