If you have a 273, no need for special parts. The heads are good, They are closed chambers flow like 360 heads with just a bowl clean up. Get a set of stainless exhaust valves, double roller timing chain, and a cam and matching valve springs. The cam is the most important since too big a cam shifts the power curve higher in the rpm. Early 273's had a 3.09 first gear which also gets you started quicker. I always liked the 65 Commando exhaust if using the stock exhaust manifolds.