Hey everyone I’m new here but get on all the time looking for information but now I could really use some expert advise. I will do my best to explain what is going on with my truck.
First let me start with I own a 1998 Chevy 3500 LS crew cab with the 454ci. 7.4L 2wd and it has just hit 160,060 on miles. I’ve had the truck since it had about 40,000 miles on it. I’ve maintained it regularly as should and babied it 90% of the time. As of recently I’ve noticed some changes in the way she is running and it’s been taxing me mentally and financially trying to figure it out.
Truck starts fine hot or cold (live in Florida) sounds good other than a small exhaust leak in magnaflow mufflers. Operates great for first 5-8 mins until it gets warm. Noticeably different in performance. Feels like she’s pulling a load in 60mph head winds and she dosent accelerate like a big block should or as she will when cold. I feel like it’s either not getting air or fuel correctly or it’s not releasing exhaust as should. I will add the truck does NOT overheat or surge or stall out ever. I get a lot of black out the cooling system even after it’s been flushed. I’ve tried just about everything and would like some advice as to what next.
Things I have done as of now. New fuel pump and filter. Fuel pressure test indicates 60 psi, I replaced radiator, all hoses including heater hoses. I’ve changed thermostat, pre cat o2sensors, both temp sensor and map sensor, new plugs set @ .60 and 7mm plug wires. Optima red top battery shows a steady 13v. I’ve tried to psych the EGR valve with 100% blocked and 70% blocked. I’ve flushed the cooling system a dozen times using Prestone treatment. Bypassed Heater Core and replaces MAF sensor. Installed new electric fan and water pump and fan clutch and even made sure the fans are blowing right direction. once again I am not over heating but feel as though something is not circulating properly after warm. The engine is thrown no check engine light. Please someone tell me what to do next! No water deposits in oil and no leaks visible other than a hot spot by heater hose at manifold connector but it’s not leaking it’s just black. I have magnaflow true duels on it that are 10 yrs old or better. If any questions please ask and I will do my best to answer. Thanks in advance.

I’m trying to do everything I can before going to intake manifold gasket replacement. I got the new Edelbrock gaskets in hand just in case I have no choice.