Firstly the sensor on the JZZ30 box is a 4 pulse/rev, and going by the amount your speedo is out, I'd say the GZ20 is a 20pulse/rev.
There are a few Toyota's around this era that are 20pulse/rev.
I'm pretty sure the JZZ30 speedo sensor bolts on, where as the GZ20 screws on.
You need to convert the box to a screw on system (requires some old parts and a little brackeering).
But it sounds like you've already done that.
If the dash is dead (ie, it doesn't even read 0) then its either a fuse blown, or the dash/instruments are dead.
I've worked on a car that was dead before (just died one day), swapped over to another one, and it all worked.
It could be that when you put the GZ20 sensor on there you blew the dash up.
Fix the dash (and the wiring assuming that caused it) and your speedo should be much closer.
As for the car starting in gear, that is most likely due to dodgy wiring by the person who wired the 1JZ in.
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