I'm not certain, but to my knowledge, priming is a more recent "desired" technique. There have been many blocks painted without the use of primer. I suspect using primer would be the better way to go and will allow your paint to bond better with the metal, but make sure to use a heat safe primer.
Your paint should be intended to spray on. If that is the case, it probably needs thinned, but without seeing it I can't be sure of that. I would consider spraying it, whether you take it somewhere where there is a gun, or return the paint and get cans of the correct color. To me there are too many obsticles to get a nice paintjob on a block. You most certainly will have some "puddles" and brush strokes.
BTW, some paint shops can take container paint and put it into cans. Not cheap, but way better results without the availability of a gun.
