I see where you say you installed new umbrella valve seals.??.....Were the valve guides cut to accept these.??...You can not use umbrella seals on small block Chevys unless the guides are cut down to accept Perfect Circle type seals...Regular umbrella seals will hang up on the inner valve spring which your emgine must have...If your getting flame out the exaust that means an exhaust valve is not closing completely during the compression stroke...bent valve stem...weak spring....sticking valve stem..etc.... smile smile smile

If all the cylinders ain't in a line it ain't cuttin it..