Well that's interesting. As in annoying...

It's possible it's already been done though.
1357 bank (the '65 head):

• 7.6 bar . (110 psi)
• 11.8 bar (171 psi)
• 12.0 bar (174 psi)
• 12.2 bar (177 psi)

2468 bank (the '67 head):

• 12.3 bar (178 psi)
• 11.8 bar (171 psi)
• 12.1 bar (175 psi)
• 11.4 bar (165 psi)

They're close-ish.

I've just found the SD Performance website, I've emailed for more details, but it looks like you get the following:

• 2 matching head blocks
• all new intake seats
• all new exhaust seats
• ferrea valves
• springs, retainers and locks
• bronze guides
  
  for ~$1350

Seeing as I have been quoted £30-40 per valve seat and 16 need to be done (intake and exhaust too close together to do just the exhaust apparently) that's more than £500 just for new seats... without even buying new guides, valves etc... and this way the two heads are matched and "original". It's tempting so long as that $1350 is for a pair and not singles... Sound good?

Also, side note, my intake manifold has the casting number 9782894. Which appears to be a '66. My exhaust manifold is 9777755, which could be a '67 or '68? Boy is my engine a frankenstein !?