Yes, it can be clocked either way. Holley 2-bbls throttles are set up/arranged different to Holley, Autolite and Motorcraft 1-bbls, and this hurts accurate fuel dilivery to each cylinder when the flost bowels are arranged frontwards. The stock US log adaptor is best, as it moves the carb to a sideway position anyway, fixing fuel distrubtion to suit the way the throttles open. Does10's didn't have any problems drag racing his 250 with the carb mounted as per your picture, neighter does Crosley, Gene or Mustang Geezer. Good company to have. Jet extenders are around to cope with fuel slosh when the float bowel is under extreme acceleration.
As a counter point, Ford Argentina used to race SP 221 engines with its Holley 350 cfm 2-bbl set up like yours. No issues with fuel delivery under acceleration.
On the other hand, Ford Australia used to race I6's with the 2-bbl carb mounted far out on the side, with its float bowel mounted forward. No issues despite technically poor fuel distribution figures, typical of any I6 installation that has the Holley 2-bbl accessing fuel non uniformly from one side. V8's don't have issues when mounted with the float bowl forward, but I6's do. Its technically wrong, but Ford Australia with all its 2v 250's and Chrylser Australia with all its 2-bbl and 4-bbl 245 and 265's mounted the csrb the wrong way, yet it worked perfectly.

so the float bowel faces the front. It often is on the overseas Aussie 2V 250 and Argentina ME 188 conversions. But then the standard 2300 series linkage is hard up against the rocker cover. Some guys turn the carb backwards towards the firewall like the early tripower Holley 352/390, 406, 427 and 289 engines.Unlike the Weber DG series and Holley Weber 5200/6500 series, the Holley 2300/4150/4160/4180 series is best mounted parallel to the crank on six cylinder engines, or the "rooster tail" of atomised fuel will be poorly distributed so the back cylinders get too much fuel and the fronts starve, as the air fuel mixture has weight and inertia as it tumbles down into the log head.
The only pro for forward or backward mounting the float bwel of a 2-bbl Holley on an I6 is under off raod conditons, where fuel can spill on certian versions. The competition float bowels and lemans float bowels and jet extenders which Does10's used on his log headed version of his Turbo 250 sort those problems out, and its not normally a problem.
The throttles on the Weber DG series and Holley Weber 5200/6500 series tip into port, but on the Holley 2300 series it is biased to the float bowel. 4-bbls are biased to the front float bowel on primary, and the back float bowel on secondary operation.
There are other reasons, and the 1972 2V 250 and 2006 onwards Classic Inlines Aluminum head have been designed with enough plenumb area to rectify the flow bias, but it still exists anytime you use a Holley 2300 2-bbl. The Holley 2305 series was released to eliminate this characterstic.