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I'm going to switch my intake, performer dual plane in place of single plane.
Engine is mild 350 (1970's) with iron world heads.
Which fel pro gaskets should I use ?

u are going to duel plane?
with irom heads....

Sensible move....and if a street car, think serious about cast truck rams horns
Get all the 'go gear' (the stuff that makes the power range, heads manifolds, headers, carb, cam etc) all in the same brakect....and if bottom end it becomes a fun car, and ecominal, and a pain in the a55e because the end lets loose when u dont want it to ordinary driving.
what heads?

this one http://www.summitracing.com/parts/FEL-MS90322/?rtype=10 but toss the end gaskets and use a thick bead of silicone instead.

yeah....
To make sure put a very thing bead on both surfaces, and spread to 'wet' the surface.
Then put the good thick bead along the block, a fine bead on th valey cover....then let the surface just tack off.

And thread sealer (locktite plumbers liquid teflon thread sealer) a few drops on each of the threads of the valley cover bolts..
They dont all need need it but that way u dont miss the ones that realy do need it.

A light smear of engine oil on the carb and carb to vallley cover gaskets stops them sticking.