The easiest "right now" swap would be the LA 318. They have not been made new for quite a while, so it may take a while to find a used engine which is OK "as is". Also the smog era 318's are pretty feeble.

A Magnum 5.9/360 would be a nice big step up in power. Depending on how long you need to run it before the Gen III swap may be worth it... plus used engines should be more readily available. MA had a good series on "warming up" a 5.9 for car use.