If you are going so far as to replace the cam, it would be not much more to pull the heads to check the gaskets, and maybe put in better ones. If the PO used the stock Felpro out-of-the-box head gaskets, that will drop compression ratio almost 3/4 of a point over the original .021" thick shim head gaskets, so it would be good to get any such gasket out and put in the .028" 1121G's.
BTW, what head casting PN's did you find?
While there, take a pix of the piston tops to see what you have in there. Then the SCR can be better determined and a better cam choice made. I know I would do this in a heartbeat with such unknowns in the engine at this point.
I would think that is too much duration for a Low compression 273. Your Dynamic compression will fall into lawnmower country, and acceleration below 2800rpm will be measured with a sun-dial,lol.