318 running with low power - dyno

If you order that intake for a set of magnum heads you'll need to have the heads drilled to take an LA pattern...unless Weiand started making one for magnum heads with the straight up bolts.

OP, follow the advice above...diagnose where you're at, what your actual cylinder pressure is, your timing (initial, total and curve), ensure your lash/preload is properly set, etc. You may need to get back in there and either advance the cam or possibly (I know this sounds backward, but) swap to a smaller cam or possibly swap to heads that haven't been messed with.

There's any number of reasons why you're low on power, but start with basics as mentioned by those like krazykuda, bigblockmopar, marcohotrod...

Compression
Cam timing
Ignition timing
Mixture (wideband tuning if available)