The 270 gmc could pull a 3.38 over drive with 26 inch tires. it won't be an ideal setup with 30 inch tires.
Just for giggles here are a few rpm/speed calcs with the 3.9 rear and .72 od and 30 inch tires.
55 mph @ 1730 rpm
60 mph @ 1887 rpm
65 mph @ 2044 rpm
70 mph @ 2201 rpm
100 mph @3145 rpm
108 mph @ 3400 rpm, the maximum reccomended rpm of a 270.
You really can't do much better than that! Just a note, tires can vary by 1 inch or more of their advertised diameter, usually on the shorter side. 1 inch shorter tires will add about 75 rpm @ 70 mph. This setup should be both agressive off the line and comfortable at cruising speed.
