That's a big enough cam to possibly be an issue with power valve size. Holley says to go 50% or slightly less on power valves than engine idle vacuum. So if your idle vacuum is 10", then you'd want to be at a #5 or maybe #4.5 size. I believe those 450's came with 4.5's? But you need to know your engine vacuum before changing anything.
I drilled right in the middle in the bottom and fitted a grommet and valve. Drilled an aligned hole in the top and JB welded a hose fitting. A short section of hose was installed and when the upper and lower were bolted together, it all worked fine and was mostly out of sight. You can add a shield under the bottom to act like a baffle in a valve cover. Downside is 2 holes, but many old TRs are modified anyway. Easy to weld back up. I also tried those 450s on mine (SBM) and hated them. ended up building a matched pair of 600s and it was much happier. A BBC should prefer more and have either mechanical secondaries with squirters or vacuum secondaries. Mine developed ~12 inches at idle (stock engines are ~18). Ended up with 5.5 power valves. Read up on tuning holleys and pay attention to air bleeds, synchronization, and gaskets, because you will be in them setting up at least a few times. On mine, I set it all up "square" meaning everything was the same front to back and side to side. BBCs have long/short ports and may need a staggered setup for the last 5%, but get it close just going square. That's it right behind the Offy script. Ignore the throttle in the pics, it's bracket isn't attached here.
Put the 4.5 power valves in. Helped alot. Still some idle fluctuations. But very little. Drove to hot rod reunion.
Glad to help. There are 7 circuits in carbs. You can probably ignore the choke, so 6. Get idle happy, then work on transfer. Cruising on mains will be a big triumph. Your enrichment is probably already close now. Float is something you can usually set and forget, but can mess up the others so keep an eye on it during this. Accelerator pumps have a lot of adjustability, and may cover for weak transfer. This will help both MPG (not too much fuel) and driveability (no lean bogs). it will often be the most time consuming. Secondaries require hard pulls, then shutting off to read plugs to dial in. If you have access to a dyno and a sniffer, you want slightly richer than an A/F ratio of ~14.7 to 1 (perfect mix) at idle to slightly lean at cruise (15-17) and under heavy load you want rich (12). With a big cam and tunnel ram, the idle is more about easy start, idle that stays in adjustment, and not fouling plugs. This will be the final adjustment and will need the most fiddling once everything else is set up for street driving. If you are bracket racing, the jetting will also need to be swapped for conditions (altitude, moisture and temps.) For class racing, everything will need to be optimized. Log books are required here, but it's a good idea to keep written records in any case. Having a pair of carbs means solid linkage that keeps them synchronized, good return springs and doing everything twice. Don't forget the fuel system needs to supply twice as many bowls, even if the engine underneath still 'drinks' the same volume of fuel, once adjusted. On the one pictured, it was easier to remove the 4 nuts and take the upper over to the bench instead of pulling each carb separately. The toughest part for mine was balancing the low RPM, low vacuum mild acceleration like pulling through a parking lot (accel and power circuits) with mild cruise acceleration where you are going from 30 up to 45 mph in traffic. The power circuit would work in one or the other, not both due to the vacuum being so different at 1300 and 2500 RPM. Too much accel pump can bog too. Street life isn't all freeway on ramps and rumpity power parking. I skimmed this article and it's got good basic info. https://www.enginebuildermag.com/2016/03/understanding-how-to-tune-carburetors/
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