318 with random parts.

So I recently acquired a 318 and I think it's a good core. I am actually surprised it's a good core considering the environment from where it was for the past estimated 30 years.

To my surprise it has the adjustable rocker arms and solid lifters. The block is believed to be a 1972 for a 1973 model year car.

I took a look at the heads this evening and they are casting number 2843675 which come back as 68/9 273 heads.

The pistons do not appear original and have the numbers "3751004 O/S" stamped in the tops.

Why would anyone put 273 Heads on a 318 ??
Anyone recognize the numbers on the pistons??
Did 68 and 69 273 engines all have the solid lifter setup??

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Open chamber? I don't have my books handy And this tiny tablet sucks.