Misfire on 98 Chevy van

I'm working on a friends 98 chevy van 2500 5.0L with 102k miles. I replaced right cracked head(hole in radiator), put back together & started right up when driving it starts to clatter like timing is off, but i've checked it several times (tdc comp. Stroke). Pulled plug wires at dist. One at a time cyl.2 & 5 did not lower RPM. Compression on cyl. 2 is 160 &cyl. 5 is165. It does show two codes P0300 misfire & P0452 fuel tank pressure sensor low voltage. I am running out of ideas any help will be appreciated.I pulled fuel injectors and cleaned, I also switched injectors around but #2 & #5 still don't seem to be firing. It does seem a little smoother when I take off, but when I get around 30mph it really starts clattering & won't go much faster without the engine really jumpng.