You may be able to get the valves to fit with some cutting, but that doesn't always improve flow. In fact, in some of the later 1.94 heads an increase in valve size without porting actually hurt the flow numbers.
283 heads are not a good idea on a 350. They will increase your compression, but they'll kill your overall horsepower by seriously constricting your flow at anything above probably 3000 RPM, so you'll make no power with them. Compression does not equal power, its only a necessary component for lots of airflow/fuel mixture. If you don't get any more mixture into the chamber, overcompressing the normal amount of fuel/air charge will likely create nothing but some pre-ignition, pingng and detonation.
If you're planning on using N20, you want to make sure you don't have a crazy amount of compression anyway, 9 or 9.5:1 is probably plenty depending on your intake runner size and cam. BTW they make N20 specialty cams too, check out compcams.com.
Hope that helps-