Crank harmonic damper ProRace – which belt?

[PhilBell]

Edited because I got a couple of facts wrong.

I've fitted a ProRace crankshaft harmonic damper to replace the delaminating and failure-prone standard one on my 1962 3.8 E-type. It requires switching from the original toothed single-V profile belt and pulleys to the double-V (or W-profile if you prefer) design fitted to late 3.8 E-types (from engine number 5250). So I've changed all of the pulleys to late 3.8.

My problem is that the late 3.8 S1 belt (c19524) is labelled as 1010mm outer length but the only ones I've found for sale are in fact 990mm. It's significantly too short to go on and forcing it is a bad idea because if it's too tight it will wreck the water pump and dynamo bearings. The next size up is Mk2 3.4/3.8 (c19525) labelled at 1050mm and is too long – the tensioner wheel and belt foul on the radiator bottom hose. The 4.2 belt (c24291) is longer still, labelled 1100mm (but actually 1119mm). As you know, there's not much adjustment on the E-type dynamo strap.

I've measured the run of pulleys on my car at 1020mm, so what I need is a double-V belt between 1020 and 1030mm, maybe 1040mm at a push.

Anyone solved this one?