But Shimano 7 and 8 are effectively interchangeable even though there's a 0.2mm difference in cog pitch - 1.2mm at the far end of the range.
The difference between typical six and seven spacing is more like half a millimetre per sprocket, too much for the floating jockey wheel to accommodate after just a couple of shifts. It's also sometimes variable across the block on pre-indexing (and even some later) freewheels, which can be a nightmare. Trust me, I've tried it.
There are indexable 7-speed freewheels around, and that might be a way to go, but freewheel hubs are always getting bent and broken axles. If the bike's worth spending money on, a cassette hub is the way to go.
In general though, the real advantage of indexing is that it lets you put ratcheting shifters on your brake hoods. Friction-shift downtube levers work fine, and are a lot less hassle than trying to piece together a bastard indexing system that, after all the effort - won't actually work any better. Friction shifting isn't anything to be afraid of.