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Shouldn't be a piston-to-valve clearance issue on an 88 engine, but it's always safer to check clearance if you're running more lift and duration than a stock cam. The ptv clearance issue is more pronounced in the 86 model, since pistons are true flat-tops with no valve relief cuts.
GT40 heads are easier to use because the angle of the spark plugs are same as your stock heads. GT40P heads changed the angle of the plugs and it will be hard to use with stock headers. A couple of the plugs will be hard to remove and you may burn plug wires. Shorter plugs and 90 degree plug wire boots will get you by, but gt40p specific aftermarket headers are best. Several other aftermarket headers provide varying degrees of extra clearance, but 90 degree wires are still recommended. Some folks run them with headers that don't allow enough clearance and just remove or loosen the header when the problem plugs need to come out.
It's also my understanding that the P heads offer a slightly higher compression ratio, which is a very small advantage. Is it a noticeable amount? Eh...I doubt it, but it might have a placebo effect lol!
Gt40 heads are harder to find than gt40p, so keep that in mind. Another budget option is to find an early casting ford heads (casting numbers D0 or C9... which translates to 1970 or 1969). These heads were on passenger cars and trucks from 69 to 72(ish). Actually, I think ford put them on trucks even later than that. The 351w version will most likely have slightly larger valves and will work fine on a 302, the only difference being the head bolts are 1/2" instead of 7/16". You can buy a bushing set to take up the difference. They just drop into the head bolt holes before you insert the bolts. The early heads were pre unleaded-fuel times, and as a result, won't have hardened exhaust seats like later engines. You will often find these heads with hardened exhaust valve seats already installed, along with screw-in rocker studs. The older hot rodder ford guys might have a set sitting on a shelf collecting dust. I had one set given to me, and the other two sets I have were $100 and $150. All 3 sets had hardened exhaust seats, screw-in studs, and extensive port work already done. I also have a set of 289 heads done in similar fashion, which will work, but require some slight modification.