L518 is better suited than L515. They are both anaerobic flange sealants (cure in the absence of air) which are applicable for flange gaps < 0.2mm, so if you have such a smooth joint then L518 will be the more sutiable of the 2. However, for intake manifolds, L587 Blue Max silicone is a better candidiate with up to 6.35mm gap fill. All this data is aimed at OEM so for older restorations where things are not so exact, maybe a silicone is more robust. Send me your email address & I'll send you a product catalogue, have a good read through the attachment & you might get a better understanding & choice
I would post it on here but dont know how to attach a pdf doc