oh yeah,
definitely go with the 600 Holley, that 390 will be a restriction.
find a good used 600 vacuum secondary and use the center hung float bowls off of the 390cfm with the carb mounted sideways like that.
best to find is a carb stamped, List:1850-1, -2, or -3, and get a 4150 to 4160 rear metering block kit, so you can use real jets in both ends.
are you allowed to remove the choke blade and linkage and remove choke horn?