Usually doing this on random old road cranks works without issue. You may find it's necessary or helpful to go to a front derailleur that matches the chain width. The reason is that if you run an FD that has a cage inner gap width that's much wider than needed for the chain like you would if you're pairing a 5/6 era FD with a 10 speed chain, you can run into the issue where for example by the time you have the FD high limit out enough for it to shift smoothly onto the big ring, the gap on the other side is too great and the chain can drop. Personally I would skip right to doing a matching front derailleur.
The chainring spacing doesn't really matter in almost all cases when it's a friction shifter like this. It does mean it might not work if you went to STI etc, between the spacing being weird and also no shift aids on the rings.
If you want to do this and know you don't need to mess with anything on the crank/FD end, just do an 8-speed cassette since you're already happy with how the crank and FD are working with an 8-speed chain. The other thing to consider there is how happy you're going to be shifting friction on the various options. Friction 9 and 10 speed isn't necessarily a problem and it can work okay, but it's not to everyone's taste. It also depends on how easy it is to make small adjustments with your particular friction shifters. The best ones are good about that (Suntour Power Ratchet is the poster child) but many will have you moving it in bigger chunks to overcome the stiction than you want for that purpose.