Compare the size of the ports. If the 350 manifold has ports noticeably bigger then the 283 heads, a step will be created that makes for extreme turbulence in the head passages. You would need to port-match the heads to the manifold runners. Not a terribly difficult task, but there's a risk of hitting water or grinding through the head if you haven't done a port-match before.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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