73 340 with 051 heads

In general, no matter the head, they all flow pretty much the same and all port out very well. The best way to upgrade the head is starting with quality valves. 2.02/1.60 sizes are the best bet in most applications. Seldom do I see larger in these heads.

Once the larger valves are in, the next step is to bowl port them. The area under the valve needs to be expanded to allow the valve size increase to be effective. How far you port the heads there after should be in line with the power goals of your build.

Getting another set of heads to build while you drive with the old heads on is fine IF you don't mind another set of heads laying around and the extra cost of getting said heads. What ever they may cost.