It sounds like your seats are recessed too deep - I have standard SBC valves in my 270 head and I altered the height of the rocker arm to get the valve geometry correct.
I ended up lowering the rocker shaft about 0.050 inches I believe (can't remember) and used Crower 68301-1 springs
68301-1 (per machine shop measurements)
Spring Rate 378 lbs/inch
Free Height 1.98 inches
Installed Height1.77 inches
Open Height 1.34 inches
Closed Pressure 90 lbs
Open Pressure 250 lbs
The motor has been to 5000 rpm several times with no valve float and when I removed the head and cam at 20,000 miles, the valves and seats looked great.
