I just noticed this on the register_block_type() page:
acf_register_block_type( $settings );
$settings
(array) (Required) Array of arguments for registering a block type. Any argument from the JavaScript registerBlockType() function may also be used.
So if you normally add your props in registerBlockType(), you should be able to add them when you register the ACF block.
However, as a person with very limited JS experience, I just add any additional classes in the block template code.
