My '31 had all the wood in front half of car replaced perhaps 50 years ago with homemade, pretty rough stuff. The only fixed guidance I have for setting front door post things in right locations are the 4 mounting bolt holes in frame, and the windshield header bar.
I made new sills/cross-sills and have bolted them snugly in place to frame. I have 2 new posts made to pattern of cowl sheet metal/door profiles, and they seem pretty close. I have established fore/aft location of cowl using frame mounting bolt holes and angle iron on cowl. Top of post location is set by windshield cross header, and by roof rails/hinge bracket etc. Remaining question is the in/out location of bottom of front posts?
It appears under dash cross piece may have been altered or moved slightly, so I cannot necessarily rely on its ends for locating middle of post. When I place posts at the moment,passenger side post sits about 1/2 to 3/4 inch further out from edge of frame than the driver side suggesting some tinkering/adjustment is required before final screwing of post/sill brace in place. When I set door in place on driver side the hinge pins are vertical. When I place rocker panel in place, it appears driver side curve line is smooth along length of car, but passenger side is not a smooth curve, suggesting the bottom of door post should be pushed in further, 1/2 inch or so, which would mean shaving perhaps 1/4 inch off end of dash cross piece or off door post or both. Then there is question of the screw hole locations for bolting the steel dash, and for the under dash brackets. A long note, but any advice on how to tackle this in the easiest manner?.

If the door hinges are vertical and all other parts line up on drivers side it is likely pretty close to correct location. The other critical parts to line up are the top, rear door post and quarter panels. Once one side is in alignment and the body sheet metal center line matches the center line of the frame just make the passenger side match.

Thanks for the advice Chipper. I have car leveled on 4 jack stands under frame, I'll establish C/line of frame today, measure driver side locations of key pieces, ensure windshield cross piece is on same center line, and try to bring passenger side to reasonably close match.

Gunsmoke, Have you looked at Bill Barkers 31 site yet? My wood kit pictures are there under "Wood". It is a coupe but the cowl area should be the same. I don't know if that would help to see anything or not. Good luck with your prject.

How do I find Bill's '31 site?