Hi all,
Thank you everyone for the detailed advice.
In response to some of the questions:
The block is an original iron 427 S/O ('67 I think). The heads and intake are aluminium Edelbrock items. The engine was built by Bill Parham at Southern Automotive, who also CNC ported the heads and intake, which were new.
I don't think there is a porosity issue because the leak only manifests after a period of time e.g. 1-2 years.
I used the silver/grey RTV on the head side of the gaskets only (I read somewhere that was the correct thing to do, but not sure why
).
I'll pull the manifold and check all faces for straightness as suggested.
Opinion seems to be split between Cometic and Felpro gaskets. I've used Felpro on two previous occasions, but not the ones with a steel core. I've also tried Mr Gasket.
Thanks again. I'll report back when I've pulled and checked the intake.
Paul