HELP 350 sbc runs like crap and has no power
Okay so I have an 86 firebird that I swapped a 350 into, this 350 has a stock cam and hydraulic lifters and stock internals, it’s got aluminum heads with 75cc chamber 68cc exhaust and 180cc intake runner, aluminum 4 barrel intake with a Holley 750 double pumper, this motor is all bolted up to a manual 5 speed (not sure if that matters but oh well) so what’s happening is it currently runs like crap at idle and has next to no power under any kind of throttle, I’ll start it up and it’ll fire right up but it idles VERY badly coughing and hiccuping (no backfiring through carb or exhaust) and then when I get on the throttle it won’t rev high at all it won’t even pass 3000 rpm, if I let it sit there and idle it’ll slowly start running worse and worse and dropping in rpm slowly after about 15 seconds of idling it would be almost idling at 2-300 rpm (which doesn’t even make sense to me) and then it would eventually die. it has a brand new MSD distributor and all the wiring is new, keep in mind I had a 350 in this car before running and driving but then that 350 blew up so I replaced it with the current one, I did not change a single thing about the wiring or fuel system when putting this new engine in, so here’s all the things I’ve checked, the timing is about where it needs to be (I don’t have a timing light so it’s just rednecked lol) I advanced it as far as I could until it died and then retarded it until it died and no where in that spectrum would it run anything close to right, I have all new plugs and wires too, it also has a nice healthy spark, so eventually I ruled out timing and spark, I have a brand new mechanical fuel pump on it that was on the other motor and working perfectly in the car, it’s getting fuel for sure, I checked all the wiring for the alternator and it’s all correct and new and neet, this alternator is also brand new and was working on the car before (it’s also got fresh gas in it too so it’s not an old gas issue) I then moved to the carb I tried tuning it and no matter where any of the adjustment screws are or where the idle screw is adjusted to it makes no difference whatsoever, I checked to see if the floats where stuck open or close and they where moving freely and no gunk was in there either very clean float bowls, I checked for vacuum leaks and there where none, the only vacuum hoses that are in use is the vacuum advance for the distributor and the brake booster hose, I made sure my valves where lashed correctly (tightening the lock nut until there was no rocker arm wiggle or pushrod movement and then went half another full turn) at this point I am stumped and can not figure out what is going on someone please help because I’ve been trying to figure this out for about 2 weeks straight now and can not figure out what’s wrong here