Changing intake gasket on a 170, any tips?

A vaccum leak will typicaly cause bad/rough idle and stumbling off idle and light cruse. Ususally with open-throttle, it will be smoother. It would help if you tell us more what you mean by open throttle.

It certainly won't hurt to reseal the intake with some RTV; don't use the CopperKoat. It is just not the direction I would go. Let us know how that does.

I would be looking for something causing a lean condition at full throttle, like a fuel restriction, plugged fule filter, ported vaccum line leak, partly plugged carb, low float level, etc., or a distributor advance issue. Kind of a wide array of possible issues!

Does this distributor have the orignal points in it?