To use A14/A15 head on A12:

A12 type
* if fitting to an old-style A12 block (1970-1973), create the oil passage needed for the head
* if fitting to a new-style A12 block (1974-1994) it simply bolts on

Exhaust pipe
* A14/A15 uses 3-bolt exhaust flange. Most A12s use 2-bolt pipe

Manifolds
* A14/A15 head might be round-point, or oval port. Get matching manifold set
* Head might use water-cooled manifold, so get matching manifold set
* use matching intake & exhaust manifolds. Divorced exhaust manifold won't fit Heat-riser type intake, and vice versa. So get a matched set

Chamber Type
* Some A14/A15 have a quench head, meant to be used with offset-dish pistons
* Late A15 (some 1982-up) have a large chamber head 35cc compare to typical 31cc of early heads. This is enough to cause a small loss of power unless fitted with small-dish pistons