witch are the best flowing 273 heads#

I think the problem with the magnum heads would be more than valve size. The magnum heads have essentially the same size ports as the 340/360. I think unless you were building a really nasty little 273, the port size might be too large with the magnum heads. Intake velocity is a balancing act. Too big a port and it slows down. Too small a port and there's not enough flow. I'm no cylinder head expert, but that's my understanding. ....and I know first hand about the port size of the magnum. I have magnum heads on my little 318 in the shop floor. I measured the ports to see if I needed to enlarge the openings for the Eddy air gap. The port sizes are the same. Maybe IQ52 will chime in. He's brilliant.