306 gti manifold

[C.A.R.]

The exhaust manifold I assume he means. The 1.8 XU7JP4 standard exhaust manifold wasn't built with performance in mind, it's just an unequal-length manifold designed to take up as little space as possible - and yes, they will corrode.

I believe you're looking at a few bolts and some which are studs in the head.

It's also worth noting that your standard exhaust system won't marry-up, since the GTi6 exhaust manifold is physically bigger / longer and reaches further back than the 1.8. Your current one will be too long.