As mlnielsen said, 54.5 acording to Ford. But basically being the same, as far as performance capabilities go, I'd sell the Hipo heads to someone restoring a K motor. You can get a fair amount of money!!! Then use that money to freshen up a plane jane set of 289 heads and have money left over or buy some after market heads.
It's a waste of a Hipo head to go on a standard rebuild IMO.
I agree with Huskinhano. The only real difference between the Hipo heads and the regular heads are the screw in studs (a real easy mod) and the spring pockets, which really don't buy you a whole lot today. I'd put those pigs up for ransom, or shelve them with that idea for later. A set of CNC machined regulars will kick a whole lot more @$$ out of the box.
I've thought that would be the way to go all along. I have a "Heinz 57" car, with absolutely no need for these "pure bred" heads. If someone restoring a K code engine wants these, I'd entertain offers. They are very freshly rebuilt (less than 200 miles on them), and for some reason they have triple valve springs. Seems like overkill to me. Although, I don't know what cam is in this thing either. Nothing like knowing exactly what you have, right? ::
I'm thinking of doing a "blkfrd"-style 331 stroker. At least if I start over, I'll know what is inside.
Figure around 10.5 to 10.7:1 with flattops. A bit much for today's fuel...*G*
Other opinions are good advice. Figure between 400-600.00 for the set if they're in good shape.
If building stroker, better to go with larger chambers; compression can get too high for pump gas in engines with the early heads. I like compression but most guys don't want to run racing fuel to utilize it.