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