I replace the head gaskets on my 1979 Chevy pickup 350 engine and now the truck wont start?

I replace the head gaskets on my 1979 Chevy pickup 350 engine and now the truck wont start?
did u get no.1 piston up on compression stroke if not u may have timing off .could be valves r out of adjustment. does it backfire back thru carb.if so ur out of time