ive got a 1980 chevy c-10 p/u with a 250 straight six and want more power and torque.want to know how hard a 350 switch would be.please help  

This should be an easy switch.  Hot wire to the v8 distributor, a little throttle linkage change and it should bolt right in to your motor mounts. You might need a bigger radiator and new exhaust while you're at it but that's about it.  The question is are you looking for the cheap way (pic n pull motor) or buying a new motor? Do you have a 4spd or th350? Stay with a carb or fuel injection like a gm 350 ramjet?  An rv cammed 350 with a 4bbl carb will give you plenty more hp and torque over that 250. A gm goodwrench motor just can't be beat.  It's new, cheap and comes with a good warranty. Go with the 260 or 290 hp model.  

the hardest part will be changing the frame mounts, other than that it's a pretty straight-forward swap.  

I just swapped a 250 to a 454 in my 67 chevelle. Frame mounts if different can be found on e-bay cheap. Not so easy to swap but well worth it.