I replaced the head gasket and she started right up but it's backfiring bad and sputters when accelerating. When I get up to 60 it looses almost all power and then shuts off. I checked the timing a couple times but it looks dead on and everything on the air box and fuel lines check out fine. I'm still convinced the timing is off ever so slightly and is throwing it off... Any suggestions??

 

Make sure you timed it 1st cylinder on the compression stroke with the cams lined up. Sounds like timing is off.