Okay so heres the deal, im new to GM im mainly a toyota guy and ive never had any issue like this with any vehicle ive owned. But this is an 81 half ton 2wd and it has a 305 from an 86 camaro in it but i swapped out the top end for some forst gen heads and intake off an old vette (no particualr reason just what i had) so that i didnt have to rune the throttle body. so, it idles just fine, starts right up, and doesnt diesel or sputter but when i reach cruising, 55-60 it would randomly start to lose power and slow down to around mybe 30 and wont accelerate any faster and in fact the more throttle i give it it seems to slow down more and the exhaust goes from a normal small block to sounding like a diesel with the jake brake on. this problem use to be intermittent and i chalked it up to old fuel and filled it with a full tank of the high octane and itr was fine for a few days but now i cant get a mile from my house. any advice would be welcome i need this rig for work asap

 

sorry about the spelling, ill be sure to proof read my posts better in the future