I'd put them on & run them like my first car said . I have a set of 69 model 4 barrel heads on a 351w I put together years ago . A bout all i had done to them was a 3 angle valve job, hardened seats ,bronze guides and at that time the guy I purchased them from sold me some teflon valve seals I had never seen before( probably 35 years ago )the guy in the machine shop did not want to do the work needed to install them,but he did . Put them on a long block I purchased that had dished pistons & they have been fine all these years. I suspect your Mustang would be a lot easier to change heads on than the van my motor was in . Who know they may turn out fine ,especially if you put some boost on it .You can always put them on and run it for a while and see how you like it while you're saving up for something even more fun because heads are kind of expensive but they're not terribly difficult to swap.
Somebody on this forum had a nice set of heads for sale a while back at what I thought was a cheap price , check the parts for sale.