Can report successful completion of this upgrade, slowed by needing to replace rear cylinders whose nipples snapped off (again).. Great Varley development work, thanks chaps.
Works fine with rear drums -locks up fronts first. I have Punto ELX front discs and callipers fitted and this feels like a better upgrade than that was.
Pedal travel is much shorter, power is immediate and as usual if I had to fo the job again it would be much quicker now knowing what's involved.
A couple of minor observations for others considering this:
The Lexus master cylinder has a couple of clip mounts cast into it and I used the handy front one to mount the tee piece splitting to the fronts.
Have not yet managed to get the low fluid warning sensor to work again but that looks possible with the right Lexus electrical plug. The reservoir has a lower outlet on the side which I plugged with a bolt and some glue.
My spare wheel at 14" didn't fit under the bonnet anyway but if you're thinking of doing this the larger vacuum cylinder does stick up further and may stop the spare fitting but I haven't checked this. The rear pressure limiting valve also needed to move for the same reason.
As Steve said - this is well worthwhile, cheap and fairly easy. Very glad I took their advice.