Frankly, it is not cost effective to use rebuilt heads, especially since
new are not that expensive.  Used heads are all 10 years old or more,
corroded, fatigued, seats brittle, guides worn, valves worn, etc., etc.
Even if you rebuild all this and the head ends up leaking, the $100 cost
of another gasket set plus one or two new heads would put you way in the
hole as opposed to just putting new heads on in the first place.

These alloy heads are simply not as durable as old fashioned cast iron
or as strong as modern in-line four cylinder alloy heads.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT