I was curious about the Lexus GS450H possibilities so I did a little more surfing. My rough understanding is as follows.
The Lexus ~2006-2011 GS450H is RWD hybrid luxury sedan and uses a 6-cyl 3.5L ~230 HP gas engine, mated to what looks like a normal CVT automatic transmission, to a 3.7ish rear axle. Inside the transmission are actually 2 electric motors and a couple planetary gear sets which boost the performance to the ~340HP level. The electric motors are driven by a separate "Inverter" or motor controller box (with water cooling ports). In the original setup, the gas engine ECU talks to the Inverter over CANbus-like connections.
If you surf around a bit, you'll find the name Damien Maguire keeps popping up. As far as I can tell, he's a brilliant and prolific hobbyist who's glad to share everything he's doing. He's done numerous electric-only conversions, including locking the input shaft of the GS450H transmission and using it for full electric in a big BMW. Here's one of his shorter videos (there are many hours of more detailed videos too):
https://www.youtube.com/watch?v=F5XM6blwMlw
He reverse engineered the factory ECU to Inverter interface and designed a ECU replacement board, a "VCU" or Vehicle Control Unit, plus his own custom software to talk to the Inverter directly. The board is available in partially assembled form on ebay, and the software is open source.
From the video, the GS450H transmission looks fairly big so it may not fit in a brick along with a gas engine (hybrid) but should have plenty of room without the gas engine (full electric).
The wiki shown in one of his other videos is:
https://openinverter.org/wiki/Lexus_GS450h_Inverter