I'd look for a set of 660 center squirters, have them done by a reputal carb shop and don't look back, they were designed for tunnel rams

The 660 center squirters are good carbs, I too have a BBC engine in my jet boat designed and built by me, I have two Holley 4160 600cfm vacuum secondary’s on my Edelbrock tunnel ram, and they work fantastic. They both have the Quickfuel Technology billet metering plates on the secondary side that house Holley jets so you can be in control of your secondary fuel curve. I had also linked the secondary vacuum canisters together so they will open at the same rate as manifold vacuum drops. I dyno tuned this engine with O2’s and EGT’s in every primary tube and staggered the jets per what we saw exiting each cylinder. This has been a very reliable set up and never has had any miss under any throttle angle.