will 360 j heads fit on my 273

If I were doing my 318 over, I'd opt for redoing my 273 heads with 1.88/1.60 valves instead of the 302's I used - the early 273 heads are closed chamber (at least my '66 273 heads are) and you don't have the problem with the 302's pushrod bulge impinging on the intake port. That bulge is a huge lump in the port that needs to be massaged pretty extensively. Put some hard seats in the exhausts and be happy.

GO ahead and buy the 360 heads for another engine project and shelve them. Its highly likely you''ll end up with a bigger motor in the car at some point anyway.

Your cam is way out of whack, use something about 3-4 steps smaller, as someone else mentioned - under 280 degrees would be a good bet. My thought would be a stock 340 grind.