Going by Mr. Bonds' sight, it seems like the Ashtabula stamped-steel forks are the ones he's using. I suspect you'll fold one of those flat-bladed prewar forks offroad very quickly, which'd be a shame, b/c they don't make'm anymore.
If you watch the movie, it seems like they were on Tange TRX1200 forks as soon as they became available. I take it you want to avoid a tubular BMX fork for aesthetic reasons, but a vintage 26" cruiser fork would be entirely era-accurate/authentic.
In the end, we all gotta ask ourselves: do we want our klunks to look like a fish, or do we want them to swim well? A klunker built from all pre-war parts would look like a great white shark, but if you put it in the water, it'd probably drown.