The MP620 cam actually measures out at about 283@.050, not 275.
You didn't specify what rocker ratio you'll be running.
With 1.5's I'd use #5.

Some SR's I tweaked recently:

Lift------int
.100---72.5
.200--153.6
.300--213.6
.400--270.0
.500--314.1
.600--338.4
.700--354.1
.800--357.1

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads