That would not really be nat, other than the normal nat from your private to your public on your wan.
Are you setup for explicit proxy - ie your browser pointing to the proxy or just transparent. Which intercepts http/https normally.
Normally if you just want to allow access to specific ports outbound, you would do that on the lan rules. Be default the rules are setup to allow anything from lan segment to go to any port outbound.
You create specific rules to allow http, https, pop, smtp, etc. And then create a block rule after those that blocks anything else.