Welcome dude.

Where you from?

Anyway, I cant agree more than what DD has said.
The a12 motor will be better to work on and much cheaper, and less time consuming.
No need for many conversions, such as changing mounting positions, g\box and crossmembers, diff, radiators etc, etc.

What I would suggest is to put an a14 head on your a12 block with extractors, an Twin Carbi or weber, and if you want, but not that neccesary,
is a a14 5speed gearbox to go on.

Thats about it.
I'm sure for the money you spend on this, you'll be quite happy with the results.

However if you want more power and have the money, go for a Lseries motor, BITCH POWER