Truck is sluggish and shifts hard, has to work hard to accelerate?

So, my C1500 was originally a 5.0 but it seems like at some point someone swapped it for a 5.7. When I first got her, she ran hard, and I was running the highways a lot. Then the fuel pump and transmission started going out and I had to replace the pump and had the trans rebuild, and now she struggles really bad in first gear, kicks hard into second and is in 3rd gear by the time I'm going 30 MPH. Getting her to go 60 MPH takes a couple minutes with me firmly pressing on the gas and im too sketched out to try to push her much faster. Oil pressure dies when I accelerate and after I've been driving for a while but when I let off the gas it comes back up to about 15-30 PSI. Valve cover gaskets leak even though I've replaced them twice now but there’s no way that’s why the oil pressure is that wonky.