After I posted yesterday, I remembered that the ignition timing on the race car was set to 10 degrees BTDC. I'd tried advancing the idle setting previously to give greater advance at the higher revs I tend to be using on the track but it hadn't helped.
What struck me yesterday was that my previous attempts were before I replaced all the ignition components.
So today I went out and advanced the idle to 16 degrees BTDC and suddenly the car can rev to 7000 (eek!) I've just taken it up the road and it seems to be going really well. It's hard to use 6000 revs and over on the road without going deaf from the noise, also at high speed the car doesn't track in a straight line on bumpy roads due to it being so low and stiff so I can't get the full potential but I'm much happier that it's all working now. I was worried that I might get "pinking" lower in the rev range with the extra advance but that doesn't seem to be a problem.
So rather than a rolling road session, I might be able to spend the money on a test session on track sometime instead. The only planned job I have left on the race car now is to finish making the new air box I started some time ago. I have all the parts for it, I need to find an aluminium welder to do a bit of work before I can continue though. Someone I know tells me they in turn know a good aluminium fabricator so I'll have to get in touch with them.
Now I need to work on some way of getting the car to and from the track without having to drive it since it's not a particularly pleasant vehicle to drive on the road. Back to looking at the van then I guess!