Will 318, 340 or 360 heads fit on a 273?

OP, since the enigne is out, pull the heads, put a piston carefully at TDC. Measure how far down the piston is from the block's deck at TDC.I bet is going to only a few thousandths if it is a 2 BBL engine. Those early 273's & 318's had higher pistons than later 318's. If so, then with a .028" head gasket, then the static CR is going to be over 8:1 with the 675 heads, WITH THOSR STOCK PISTONS. If you use aftermarket pistons, then that will change.

675 is the last 3 digits in the casting number on the head for the later open chamber 2 BBL heads on 273's. They have chamber volumes that are typicaly around 68 cc's. Look at you heads and find the casting number, it is a 7 digit number and is usually cast on the intake runner. Let us know what you find there. And if you would please, can you post some pix of the tops of the pistons and the chambers in the heads?

675's have 1.78" intake valves and were used in the earlier 318 2 BBL engines. You can upgrade these to 1.88 valves with little or not notching of the tops of the bores.

I'd do as advised before and have yours re-worked, since you just want a modest improvement in HP. A modest cam upgrade and a small carb will help. The cam selection needs to be carefully done.