Other guys on here more experienced me, but stock 4V heads will flow somewhere around 260-270 intake and 160 on the exhaust.
I had a set of heads I did myself with intake "spoons" (cross-section made smaller) that flowed 325 intake, and mildly ported exhaust flowed 200. A good experienced porter can get as much as 360cfm or more. An excellent max-ported exhaust might go 230 but I'm guessing. With exhaust plates exhaust can get up in the 260 range.
A stock set of 4V heads should easily support your target 425hp.
68 Mustang coupe NMCA Mean Street
347 NA @3100lbs
best so far: 10.52@125 with 1.43 60' on goodrich 275-15 drag radials looking for 10.20's!
67 mustang 11-teens with 351W/C9OE heads and rusted out floor pan
69 torino gt 351C 4sp
78 F150 460 running low 14's