Thanks for the good info, Lynn. I'll check out what pistons and cylinder heads I have.
But I found something else interesting. I only managed to get one exhaust valve/guide assembly out. (The others are stuck in the block, apparently a common problem with flathead Fords.) Looking at the seats with the valves fully open, I flagged this exhaust valve as OK. (It's on one of the "good" cylinders.) But check out the photo taken after I cleaned it up. It's far from OK. It's blackened and pitted. Compare it to the photo of a good intake valve face. I'm beginning to think the whole low compression issue is down to bad exhaust valves.
If penetrant doesnt loosen up the stuck valves, I'll have to make up the c-clamp tool to get them out.
Malcolm