I wouldn't bother with the old heads, any of the old heads, including Tunnel port, cool though they are.
Fact is, by the time you get the old heads cleaned up and ready to go it would cost as much as new alloy heads. Plus the alloy heads are much easier to repair in case of a problem, like loosing a guide or a valve seat. A common problem with the older heads. But if you have a set already, there not a bad choice, they got some soul, run 'em till their break and then move on.
Point is, I'm surprised how many folks on this thread discount the history of the FE and it's potential for power. If those GM big blocks were so good, if their heads flowed so much better, why didn't they set the records for that era? Fact is, the FE is one of the great race engines of it's time, GM couldn't touch it "back in the day."
While the many of those great old engines like the Buick Nail Head, the Olds, the Pontiacs (which were pretty awesome themselves) have faded into history, the FE found new life on it's way to that junkyard in the sky. New technology has brought it into the 21st century. It was and remains a worthy contender.
Consider the cfm flow of the heads as only one small factor in making horse power, there are a ton of other variables as well. But just looking at cfm flow we can estimate some basics. Like 300 cfm will support about 630 horse. Interesting that the Tunnel Ports did about 300 cfm and there is a documentation of all STOCK, period correct everything, no modifications at all, FE making a bit over 630 horse, with tunnel port heads. FT TRUCK engine heads can be modified to flow 300 cfm and there not even High Risers or Tunnel Ports! There is no question High Riser heads with some decent work will out flow stock Tunnel ports. Which means, they will easily support near 700 horse.
Theory has it 380 cfm will support about 780 horse. It's all just theory as it concerns flow and potential horse power. But there are enough examples to see that the theory often holds true. Assuming you match all the right components and eliminate the various bottle necks (no bad intake, cam, header, carbs, etc.). It's the combination of parts that speak louder than any single part.
BT heads, like an alloy block, are out of my price range. IF my side oiler detonates and cannot be repaired I would likely go with a modest 390 and shoot for about 500 horse, which is do-able. My neighbor just bought an old Ford pickup with a 390 (maybe it's a 391?), run's great! He paid $200 for it and drove it home. I might make him an offer, double his money. Potentially a steel crank!!
But my High Riser heads won't work on that engine...