Well....back in the day (like back before the GT40/GT40P heads were ever used on stock engines), the '69-70 351 heads were THE junkyard head to get, unless you could get your hands on some 289 HiPo heads. The one down side to these heads right off the bat is the combustion chambers are 60cc (actually like 60.1 or something like that) which won't provide much compression. If you had them milled down to 58cc or so, they'd be pretty nice.
Now...my car has '69 C90E heads...mine are ported/polished quite well, and have 1.94/1.60 valves. They went 12.8 in the 1/4 and probably would've gone at least 12.5 with more traction, and right now they're making 362hp/364tq on a 347. They're making pretty impressive power, but the magic is in the port work which changes everything and means you have to spend money to get there.
As for the GT40/P heads, i personally would take those over unported Windsor heads, for a couple reasons. First, smaller chambers means a tad more compression. Second, they flow just as good (arguably better but we're splitting hairs here). Third, right out of the junkyard you would actually need to replace the valve guides in the Windsor heads because they ran on leaded gas (lead was used in gas as a lubricant, and without it you'll burn up the guides) and you wouldn't need to do this with the GT40s. Lastly, they're just easier to find if you ever cracked a head or damaged one and needed a replacement. Out of the two, i would opt for P heads out of the two, but remember there's the issue with the headers and spark plug angles so keep that in mind.
Oh and one last tidbit, with the GT40s you can use stock rockers...the Windsors are stud mount so you'll have to buy new rockers and likely new valve covers to clear them. Also the Windsors use pushrod guide plates, which means you'll have to buy hardened pushrods to use them.
