340 intake leak

Positive Crankcase Ventilation means just that - getting enough ventilation thru the crankcase - pulling All the bypass gasses - the cylinder bypass gasses - into the intake for cleaner emissions - lowering overall emissions. Older engines used to dump the gasses out the back of the engine compartment via a dump tube. Modified engines are not especially compliant with burning all bypass gasses. More than one breather may or may not comply with the original intent of sucking ALL the bypass gasses from the crankcase.
As for the manifold gasket problem - the gaskets can be nightmares.
The manifold to engine/head distance is considerable at the front and rear and gasket sealer is not sufficient. AND Remember the PINs. Original manifold or aftermarket???

Above ALL - Take your time. It will be worth it. Hearing the thing run properly the first time Beats the Hell out of hours and hours of screwing around because of a vacuum leak.

Gaskets (if used should be cork - thicker and stickier in my opinion.) In either case let the silicone sealant - a good thick coat - cure on the base (engine side - with or without gasket) until firm - it should still be somewhat tacky but no release when touched. Then a layer - a fresh thin layer laid on the manifold side before installation. Install manifold. DO Not get in a hurry to Clean the Excess. Let it sit and cure before cleanup or you may pull Uncured Sealant from Critical Areas. This is a pain - but well worth the time - maybe even wait a few days before cleanup. It can take a long time for silicone to cure but well worth it.
A good startup without problems will allow you to concentrate on true top startup and performance issues rather than dicking around with a problem that will drive you crazy especially when you have fresh engine components needing attention.

Hope this is helpful.
Been There Done That University student, Retired. Just Yakking.

Later,
Bcudamatt