As yet, I haven't run it for an extended period.
With the power connector were I've put it, the lid won't fit on 
I've got some cheapo thermometers on their way, as I want to keep an eye on the PSU, and CPU temperatures the first few times I run it, with the lid closed.
I was also considering adding a fan into the top of the case, but I'm not sure how much this will help...
Personally I'd use a fan if that was my rig. Even a slow fan will do as it will move at least some air about. There's a small space to the left of the floppy where some heat vents remain. I'd probably rest it in the case at 45 degrees, drawing air from the vent blowing it towards the front. If the cpu does get hot, then just put the fan anywhere... As long as the air flows towards the heat sink.
I have a fan in my A600, as mine has a ACA630 which in my mind is too warm for comfort without the fan.