273 for 318 swap advice

You don't mention what heads you have, but I'd bet they are the answer to your problems if they are 273 or 318 heads. Also, you don't mention compression ratio, but the 273 won't make much power below 10:1, where the original 273 Commando was that or more. If you know your CR, and the heads are not 360 or swirl port 318, you may be able to simply swap heads. I mention this because of your budget constraints. Otherwise, trading up for more CID is a good idea, and I'd still recommend the better heads.