Bad head gasket? I

[tino0217]

I have a 71 C10 with a 350 small block that was put in by the previous owner the engine is between 74 and 78 I recently put on new timing chain carburetor radiator spark plugs spark plug wires and I've noticed timing was perfect for about a week and then I noticed when I get up to about 60 miles per hour I can see the tachometer bouncing around. Had a friend come and look at it and he noticed that water was trickling out of the tailpipe on the passenger side when i shut it off. There's no back firing. He said it was probably a blown head gasket there is no water in the oil when I pull the dipstick. I replace the intake manifold gasket new manifold gaskets new water pump new fuel pump new alternator new starter to keep this as a daily driver should I consider putting in some Blue Devil?