holley 2280 bowl vent lever missing

Which Holley 2280 did you buy? The model # should be stamped on the float bowl to the right of the fuel inlet valve. The right model for your '65 273 is the R7890, so you'll have the right jets, ports, linkage, power pistons, etc.. I bought an R7890 new for my '67 273 in the early 1980's and just rebuilt it yesterday. As far as bowl vent parts, you'd need to buy another 2280 and take the vent parts from that one. Various versions of the 2280 were standard equipment on the Chrysler cars and trucks with the 318 from about 1978 to the late 1980's, so what you need might be in a salvage yard near you. The bowl vent parts seem to be pretty much the same from one 2280 to the next, just look for the same shape cover over the accelerator pump/bowl vent compartment.

If you do decide to rebuild that 2280, be aware of Mike's Carburetor Parts.
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He's a source of some widgets you'll want or need for rebuilding. He has a 5-part youtube video on rebuilding the 2280. Unfortunately, it starts with a disassembled and cleaned carb, so no disassembly hints. He demonstrates re-assembly and that's pretty much OK, except that, as far as I can tell, he installs the bowl vent rubber valve correctly, then decides it's wrong and re-assembles it backwards. I didn't find any corrections to that in later parts of his video. If you decide to rebuild, just be aware that every single gasket is probably going to be stuck tight, so you'll need to pry things apart cautiously and remove a lot of stuck-on gasket material.