Something to keep in mind, guys. Most silicone based products are not compatible with gasoline. Read the labels. Most will say, "Not for use in direct contact with gasoline" or words to that effect. Initially they will seal but over extended exposure to gasoline they will soften and turn to goo.
As far as the intake and exhaust manifolds; do a little work with sandpaper and a flat surface to make sure the gasket surfaces are flat and true then use a good quality gasket material and torque the fasteners and you won't need a band-aid sealer.
Tom