How much power from intake swap?

I ran a single 4 Offy tunnel ram on a couple of different mild 400 B engines with cast iron exhaust in a 71 Demon daily driver and it was great. Full range power increase. In my 452 RB Duster the tunnel ram is set up like a split dominator using a pair of modified 6425 Holleys. I switch between a six pack with 4412s on the outboard and a 280 CFM center and the tunnel ram with the two 2 barrels and gain half a second or about 50 HP with the tunnel ram. I was concerned that there might not be enough plenum area above the runners for even flow on your rig but apparently it is not an issue.