I downloaded a copy of the 354/HA1 gearboxes from the French R4 club forum, which was quite enlightening about gear ratios...
There actually were 3 final drives for the 5-speed box, 3,1:1 (like on 4 GTL), 3,44:1 (R4 956) and 4,125:1 (like on F6). 1st, 2nd, 3rd are exactly the same ratios as on the 4-speed, 4th is slightly lower and 5th is taller than our 4th (overdrive at 0,949:1). This one with a 3,1:1 final drive would make a really long-legged top gear, only suitable for descents with the normal GTL engine. Or perhaps would be suited to a GTL with the Hennessey hot-up kit.
The final drive on the F6s was the lowest available (4,125:1), simply because the van had to haul 400kg of load, and start on steep inclines loaded... So Lobster, you may se impressive difference by simply fitting a 956 gearbox (like Andrea's) with the 3,44:1 final drive. I guess GTL's 3,1:1 final drive may be too tall for the F6s aerodynamics, but then you already have an R5 engine and may prove itself fine...