Well to have any hope getting really good fuel economy on a V-8 a number of things would have to change and one of them would be operating the engine at very low rpm under at least a moderate load. A big cylinder at very low rpm is going to be a problem child from the standpoint of detonation, but ignoring that and A/F mixture preparation and gearing for now, what would be a practical limit to how low (rpm) you can go on a decent V-8 before there are issues to address - mechanical, lubrication, etc. etc. Very low rpm is not something we have ever looked at, so I'm throwing it out there in the hope that someone may have something to offer. Obviously tractor engines and the like can run at very low rpm under load but thats not what is needed if we are going to have our cake and eat it to.
