A lot depends on the trim of the GT42 you have and what the application is. The GT42 is very similar to the TA45 and there a plenty of TA45 series turbos bolted onto ford sixes. Both the TA45 and the GT42 use a 5/16" shaft up from the 1/4" shaft used on the T04E series.
See the attached link for the dyno chart of Anthony Brigg's TA45 turbo crossflow in his 9 second Cortina.
http://www.paramountperformance.com/customers/carimages/cortina_dyno.jpg
TA45 turbos (and the GT42) start with a 76mm turbine wheel which is a fair jump from the P-trim wheel which are only 60 odd mm( I can't remember exactly) This does give the Ford six exactly what it needs though and that is exhaust flow required to handle the 600+ hp the GT42 is going to want to make.
Are you sure the GT42 is ball bearing? I thought they were all plain bearing.
Also is the car an auto or manual. The GT42 will be fine with a stall converter around 3000 - 4000 rpm. Don't expect the car to be nimble around the back streets though.
Even with my T66 turbo crossflow, 3000 rpm converter and C4 transmission the 235/45/17" street tyres are no match for it above about 30% throttle. The boost comes on pretty hard as soon as that 3000 rpm converter locks up and loads the engine!
The amount of lag you get will also be largely dependant on cam selection, inlet manifold runner length, exhaust manifold runner sizes and exhaust system backpressure. Once you go GT42 you really have to start thinking BIG about all of these things too.
Big turbos need big volumetric efficiencies to work properly. You need big ports to fully use the flow of the GT42, you will probably need to go 1.5" steam pipe for the exhaust mnaifold runners which seriously reduces the air speeds in the runners and increases lag.
Alternatively you compromise on exhaust manifold runner sizes and use long inlet manifold runners and smallish camshafts dialled in fairly advanced etc keep the air speeds high at low rpm for good spool up but then you will choke the flow potential of the GT42 and you may as well be running a well setup T04E with a T66 compressor wheel which is still good for 700 flywheel horsepower.
