Things were changing pretty quick. In 1949 when the Olds and Cad OHV V8s entered the market, they would have been rare and expensive in boneyards. By 1955 when the SBC was introduced, availability of both the earlier OHVs had lowered the boneyard prices considerably. The SBC didn't immediately displace the Olds and Caddy engines, and every year the early ones got cheaper.
A box-stock 1949 303 Olds was rated at 135HP. The '49 Ford flathead was rated at just 100, and to get 135 out of it, a 35% increase, would have probably cost more in machine work and parts than a complete junkyard Olds. The flatheads are severely limited by their poor breathing, even with the trickest of heads (except for the Ardun and other OHV conversions, which were way beyond the financial reach of the average rodder) so the effects of camming and carburetion weren't as pronounced as they are on an OHV. In the event, the typical rodder could get a lot more streetable, reliable power out of a Cad or Olds on carbs than out of a blown flathead. (The blown flathead in the photo above, while probably using a cheap surplus GMC 3-71 blower from a military landing craft, also has water injection and would have to be running a pressure-retard ignition system...these are not 'typical'.)
Some guys held on to their flatheads because they were already built, or just because they were resistant to change. The smart rodders who were about gong fast realized early on that there would be no way for a flathead to keep up with a rodded OHV in the performance race, and they made the switch.
If you go back and read the tech articles in Hot Rod Mag of the period, you see the advantages of the OHVs catching on pretty fast, and the knowledgeable guys at the mag disseminating info on things like power-to-weight ratios, volumetric efficiency, camming vs carburetion, etc. The info was available and the leading-edge guys went OHV early.
What you build depends on a lot of decisions the owner of the represented car would have made, relative budget, year, etc., and simply how good an engineer, natural or degreed, he was.