Well it definitely a 318 that some has installed the solid 273 tappets in and used the 273 rockers and heads on. This would give a higher compression 318 set up that would have a stronger low rpm end torque curve. It may have been a situation where someone had a block fail and simply used the salvageable top end pieces. This late in the day no one can say. If it were mine, I would go the rest of the way and find a 360 block and use the heads from the 60 and the valve train you now have or at least the rockers and a new set of tappets and cam. Unless you have a good forged crank that would be a plus for the 318.
As far as the life of the lifters, that's partly in the break in and even more in the oil additive, you must use a zinc additive or replacement in the oil to keep the cam from wiping out prematurely. Or just bite the bullet and put in a neat roller system. Since the factory rockers are adjustable, all this old stuff is still cool to have and use but unless you're into the nastalgia of it there's a lot of better newer stuff to be had.
