rough idle sputters low power under load

93 s-10 blazer 4x4 4.3 (swaped old trashed engine for a 94 4.3 out of a bravada) ive replaced the cat cap rotor but it still runs rough... at a steady run if i don't have to get on the gas its fine but under a load the
exhaust gets throaty power drops and it sputters... not a huge loss of power but noticeable.
Thanks Guys
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I had a silimar problem with my 1500.
The plug wires were resting on the exhaust. Justby moving the wires it solved the problem.

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