Holley 600 carb-4160

That carb is just as adjustable as any other Holley 4160 series carb, it just doesn't have externally adjustable floats, you have to take the bowl off and bend tabs on the float to make an adjustment. Primary jets can be changed as can secondary springs, accelerator pump cams and shooters.

The secondary metering plate can be swapped for a metering block (holley sells a kit with the block, longer screws, a longer fuel tube and gaskets) with screw in jets and will then be a 4150 carb. Or, you can buy and aftermarket plate that accepts screw in jets. You can also swap on bowls that have provisions for external float adjustment if that becomes an issue.

I have a Holley 600 that has adjustable floats and never had to touch them. It also ran a 13.7 @ 102 with a very traction limited 2.2 sec. 60' on the 360 in my 68 Barracuda.

IMHO, I would spend the $240 from Summit or Jegs and get a new one.