The 66mm dimension is there to clear and deshroud the valves. On the correctly-built 736 gaskets, the holes are oval to fit the valve situation. On the 836 bores, they can be round since the head doesn't change unless you do some extra work to it after the bore job.
Using the 836 gaskets in a 736 engine will soon cause carbon buildup in the void. This buildup tends to create hotspots when running for long periods of time (more than about 30 minutes), which can make for uneven ignition or ping. It's not the best way to run, as it makes the engine feel erratic. Adding octane to the fuel (premium) helps, but then the plugs tend to foul.
If you don't want those 836 gaskets, I have several 736 size ones extra: maybe we could swap? I'm being requested to do more big-bore engines this year than normal ones, so I have some spares. And, having some of them as 836 spares wouldn't hurt.
