[HO455]

54blackhornet had same thought at first glance I did. Masking tape is poor at holding a vacuum. A piece of the correct size rubber hose with a screw in the end works as good as anything to stop a vacuum leak.

All or part of your problem may be because you don't have the choke stove installed. The stove hooks to the piece the green arrow is pointing to. That piece in turn is supposed to control the fast idle cam. (Blue arrow) If the piece the green arrow is not secured in the choke open position it flops around and causes the fast idle cam to engage intermittently.
It is easy to lock the fast idle cam in the down position with a zip tie around it and the bracket. Take the truck for a drive and see if helps.

Your factory tachometer appears to be as accurate as mine. Mine reads about 8-900 RPMs high when the engine is turning 2500.