Back in the 80's the Aussie clev heads got a lot of press in car mags as if they were best thing since sliced bread. They offer only 1 real advantage over the U.S. 2V heads, a small quench style combustion chamber. As popular as Clevor conversions were at the time, the biggest complaint about using 2V heads was the detonation prone chambers made it hard to build in any decent amount of compression and run it on pump gas. As far as port flow numbers, they are very similar to U.S. 2V heads, which isn't all that bad for a street driven 351. They are about the same as the low end of aftermarket aluminum windsor heads.
For what the castings cost, plus the work to rebuild them for a performance engine, the benefit of the quench chambers over U.S. 2V heads is questionable and certainly makes the use of 4V quench heads a more realistic option. Contrary to wide spread opinion, the 4V heads can work just fine on the street and you don't have to rev it to 7000 rpm either. The key is proper cam selection and adequate compression for the cam. Just look at the '71 Boss, the fastest 1/4 mile of any of the Boss's and the main difference between that and other 351C's was the solid lifter cam plus 11:1 compression. The '70 4V was no slouch either, and both of these engines used 4V quench heads that had the same port and valve sizes. Later 4V engines that had lower compression for emmissions regs are IMO the main reason the 4V heads have the poor rep for street performance that they do. If you really want to improve performance over the U.S. 2V heads and the 4V seem like too much port size for your plans, go with one of the aftermarket C heads like the CHI 3V.
"It's not having what you want, it's wanting what you've got!" - Sheryl Crow