A TR4A body on a TR6 chassis basically is a TR250.
the only diffreance I know of between a TR4A IRS chassis and a TR6 chassis is the mounting points for the steering rack were moved forward to accommidate the 2.5L motor. and some good modifications were made to the front lower control arm mounting points.
Sounds like an excellent project....hope It materializes into reality... I'd love to see that done. Subtle but effective.
I agree with Kindofblue, I think the TR4A had the nicest trim of the TR4,4A, 250 body style, and that body will definatly fit over the 6 chassis, where as a TR4 might need some tweaking.