Bill -
I have 3 sets of Canfields, and they work very well, more inline with the AFR205 if ported well (a little less flow on the intake side of things), great exhaust ports. The ports are raised .400, but the header will only go up .200" Now will that be a problem on a car with a lot of room, probabally not, they were a problem on the 65 mustang with stock headers, would not fit. Also look out for head bolts/stud interferance between headers (flanges too). Make sure if using studs that you use use 12pt nuts and that will eliminate any issue.
On a GT40 with Crossover exhaust might be a bigger issue since the I'll bet the .200 raise will also make the headers further apart (L/R). Since tubes cross over might be a problem, not sure. I thought about this too, but not a problem if Fran is going to do the header work. Also if you want an easy job and clean exhaust port, use the wide (3" I think) 'Outer holes' for the header flange bolts. Also keep in mind that only a couple of head use that pattern if you are thinking about switching to say AFR or Edelbrock down the road.
I also mentioned this in another thread, but Canfield oem's the heads to Ford Performance Solutions in Anaheim, CA as the 'AVENGER' series. They are the same except the very loud AVENGER Logo tattoo-ed on the ends. One set was ported by them the other 2 by Total Engine Airflow (Kentucky). The FPS heads flows a bit better, but not by much. I think they all use the same CNC program.
I have chatted with John at Canfield a few times, very helpful with any info needed. Also the 54cc heads are the 58cc milled a bit. I found this out when they polish the chambers it increases the volume a bit, and I asked why don't you start with the 54cc head and be closer to target, and they said, then your heads would be milled twice (From T.E.A.).
If you need some piston numbers for 12.8:1 331 for those heads I have them at ROSS. They do a nice job and are local. I'm pretty sure it is the same as twisted wedge heads in anycase but you should make sure your pistons are OK for these heads.
HTH with all the extra chatter!
Sandy