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Okay...I am going to tackle these questions, and then this is going straight to the "Things We've Seen Before" thread at the top of the page.
First of all, you don't need a T3/4 turbo. For a custom kit, something the size of a 14b would be more than enough power. Especially considering the fact that the 420A engine utilizes a 9.6:1 compression ratio. This means you need less boost to reach the same power level of an engine with lower compression. If this turbo can get AWD cars with 7.9:1 CR in the 110mph trap speed range, it will be more than enough power for a "stage 2" turbo kit for a 420A. Also, it is internally gated so you don't need an expensive external wastegate.
Having said that, here is the rest of the stuff that a custom kit needs. (this is a rough list...not by any means complete or detailed).
Parts: -Custom Exhaust manifold and gasket -Turbo -External wastegate and flange to the exhaust mani if using a Garret turbo -Dump tube for the external WG -Piping (turbo to intercooler, intercooler to throttle body). -Blow off Valve and a way to mount it -Intercooler -Rising rate Fuel pressure regulator -In-line high pressure fuel pump -Fuel return line -Couplers, hose clamps, and fittings for the intake piping and fuel system. -Fuel cut defenser -Oil feed and drain lines for the turbo -Oil sending unit "T" for the turbo oil feed line -Various fittings for the oil pan tap -Custom downpipe for the turbo to mount to the exhaust system -Boost gauge -EGT gauge w/ probe -A/F gauge -Something to mount the gauges with/on.
Annoying tasks: -Getting the fuel lines running correctly- from the tank, to the pump, to the FPR, to the fuel rail, to the return line, back to the tank. -Taking off the oil pan, tapping it, and reinstalling it without it leaking. -Getting the oil sending unit to send oil  -Beading the pipes so they don't blow off under boost and checking for boost leaks
As for the other questions: The 20G and T3/4 are just huge turbos. One is Mitsubishi (20G), the other is Garret (T3/4). The Garret turbos are externally gated and the Mitsu ones are internally gated. (though the 20G can come externally as well). In fact, any turbo can be made to be externally gated. Just need to remove the wastegate flapper and port the hell out of the holes.
The Frank series turbos are a hybrid of both Mitsu and Garret parts. They are supposed to be the "Best of both worlds". The Mutt series turbos are the same basic idea.

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