Rocker arms

Will 1.6 roller rockers work on my 350 with flap-top pistons and an rv-cam. Don't know exactly the lift on the cam, trying to contact previous owner. What kind of performance upgrade can I expect also.

Yes they will work. As far as numbers go for power increase I can't say for sure. If you have stock rockers now the increase will be greater, due to the stock stuff not being very accurate in ratio. Most stock rockers are not true 1.5.
Without knowing your lift you should check piston to valve clearence as well as valve spring clearence. You should be safe up to at least .500 lift.
To get the most out of it, but time consuming, first put the 1.6s on just the intakes, test it, then just the exhaust, test it, then all of them and see what gives you the best gains. The reason, which I won't be able to explain clearly, is that an engine requires a ratio of intake flow compared to exhaust flow. And sometimes while doing different mods to your engine you either change this ratio or make for different requirements of this ratio. So when you change the rocker ratio on just one valve, intake or exhaust you are changing this ratio.
And by experimenting as I said you may find a larger improvment then by just increasing the rocker ration on all 16.
Did that make sense, or just confuse you?