Think I've cracked it.
Moving the whole shifter mounting upwards wouldn't do it, as the 3/4 rod sits underneath the operating cams for 1/2 and reverse on the side of the box. I'd have to refabricate a very complex rod shape for this still to work.
So - back to plan A: redrill the hole for the reverse rod higher up on shift lever in the shifter mechanism.
No chance! These levers are either hardened steel or surface hardened. HSS drills won't touch it, barely scratched the surface.
The answer was to cut through the arm with an angle grinder, then reweld it back together with the hole in the right place. This I did, then cut and rewelded the reverse rod to suit. I did the same with the 3/4 arm and rod, to bring it closer to the casing (it was fouling the floor pan) and slightly higher.
The first pic shows my initial problem: The cross member would sit exactly where the heavily-angled rod is, level with the trans mount casting base seen ahead of this. Hence the need to gain an inch.
The second pic shows the end result, with reverse and 3/4 modified rods and arms.
The third pic gives a few from above, which shows the mod to the reverse rod more clearly.
I've checked operation and all gears engage correctly, possibly a little easier than before as the operating angles of the rods have improved. Shift movement doesn't seem to have increased notably, but we'll see when the trans cover goes on.
So - Sunbeam Tiger toploaders will fit with original-type trans mount with a little shifter rod surgery!
Roger