I just finished a set of heavily modified 074 heads for class racing. I installed deeper seats, welded the EX floor, and ported the heck out of them. They came out pretty darn good. The IN port is 340cc with a 3.6" square min. CSA and a 2.300" valve. They were flowed on a 4.600" bore cause that's what they are destined for. A smaller bore would not have made much difference. It's hard to believe this old stuff can work so good using modern techniques.

Flow numbers from my Seanz 680 bench.

lift IN / EX

.100” 81 / 75
.200” 166 / 119
.300” 248 / 157
.400” 312 / 191
.500” 354 / 210
.600” 378 / 233
.700” 392 / 255
.800” 400 / 272
.900” 400 / 280

Valve diameter:
2.300” / 1.880”

Bore Size:
4.600”

Test Pressure: 28”

IN to EX Average:68%