I've been revisiting this LS1 idea, one that had not gone away. Your statement rovercare has me very curious. I wonder what input the BECM needs from the existing ECM whether it is auto or manual? If indeed I can switch it off and have no input from the ECM, this would be another thing in my favour.
 
- The LS1 ECU would get its own VSS signal from the 4L80E so it would be a case of LS1 and 4L80E as standalone;
 - Existing VSS signal comes from the SLABS ECU straight to the speedo(dash);
 - I'd have to double up on oil pressure and temp senders for the dash;
 
- I have ACE and there is a engine speed input from the ECM. The SLABS ECU also needs an engine speed input. It would be nice if the LS1 ECU could provide this but I'd seriously doubt it. Nevertheless, I still need a tacho so I'd have to give this some more thought;
 - HDC would get thrown out as this is controlled by the Motronic.
 
The hardest part would be manipulating existing systems to work with a different engine. If this could be done easily then it's just a case of making everything fit in the chassis. The mounts for the engine and LT230 would be straight forward I think as would the driveshafts. A set of HD Ashcroft CVs and axles would make the drivetrain more reliable too. 
 
Is there anything else that I've missed or should look at?