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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.
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 .
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.
 

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In answer to the questions, these are fairly spendy pistons, and the shortblock is already fully assembled and balanced. I'd hate to redo all of that.

The old heads have supposedly been milled once. Even if they hadn't, taking them from 63cc to 58cc is pulling quite a lot of metal, and would mean I'd need to work with port mismatch as well as a very thin face that might cause problems later. So "fixing" them is out. Someone else might well find success with them.

I have been chewing on this too, but an old racer friend of mine finally nailed it down for me. These had 1.94 intakes and 1.6 exhaust valves put in. I never thought much beyond "Good, those are the valves I wanted anyway". But when they take material out to put in the bigger valves, guess what? The chamber gets bigger too. So there's my problem. If I had flattops with 6 cc reliefs, I'd be fine with big chambers (probably too high on compression unless I go with a hotter cam), but I went into this thinking they were going to be tight chambers, so I got reverse dome 16cc pistons, shooting for 9.5 or less compression. Looks like I got "less" all right! Looked to be around 8.75

Anyone have some aluminum heads lying around with 58cc chambers and smallish intake runners?

It's either that, or I buy some new, looks like...
That would explain it. Yeah, if it was my engine, I would buy some spiffy new aluminum heads with very small chambers. No fun building a low compression engine unless it's going to be asperated.
 

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I run a 69-70 351w set on my late 80's 302 with the factory roller h.o. cam and 1.7 rockers. Much like your description. They were slightly surfaced to bring down the largest chamber size on one end to the smallest, 59cc on the other end. They also have the larger Ch#vy valves. Did some one do some grinding on the combustion chambers? If you are going to get another set, you may want to go with some AFR's unless you just want iron.
 
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