I phoned Heritage today about some parts but when it came to the head bolts I have 2 different sizes?!?!?!?!

Apparently the first 300 engines produced had smaller bolts, M11 and not usual M12 ?????

Is there anything especially different about these earlier ones
Are they worth anything other than historical value
Is it worth the extra work to keep the engine as it was or do i just expand the holes in the head and bolt it on!
ATB Kris