Do you have information on how long the first Willow Run B-24Hs spent in modification centers?
No. Over the course of the war the USAAF delivery logs changed format several times. This is what they have for the first Ford built B-24H. Later page formats drop the engine serial number columns to enable recording of modification centre dates. Looking at the aircraft cards might help match dates to any modification centre stays.
First B-24H page, columns for serial, acceptance date, country and priority. Next is group I, covering grounded date, grounded short, flyable date, flyable short. Next is group II with date, short and destination. After that are columns for the block number and engine serials. The group I dates are the acceptance dates, the first 6 entries are in grounded, the rest in flyable, where the short field in group I has data it is the letter C or S, the short field in group II has no data.
Serial 42-7465 Accepted on 30-Jun-43 Priority 28 Short S Group II Date 13-Jul-43
Serial 42-7466 Accepted on 30-Jun-43 Priority 34 Short S Group II Date 16-Jul-43
Serial 42-7467 Accepted on 30-Jun-43 Priority 33 Short S Group II Date 14-Jul-43
Serial 42-7468 Accepted on 30-Jun-43 Priority 33 Short S Group II Date 13-Jul-43
Serial 42-7469 Accepted on 30-Jun-43 Priority 33 Short S Group II Date 16-Jul-43
Serial 42-7470 Accepted on 30-Jun-43 Priority 33 Short S Group II Date 13-Jul-43
Serial 42-7471 Accepted on 12-Jul-43 Priority 33 Short C Group II Date 16-Jul-43
Serial 42-7472 Accepted on 12-Jul-43 Priority 33 Short (empty) Group II Date 16-Jul-43
Serial 42-7473 Accepted on 21-Jul-43 Priority 34 Short S Group II Date 2-Aug-43
Serial 42-7474 Accepted on 9-Jul-43 Priority 33 Short S Group II Date 13-Jul-43
The second delivery log page has a different format, Serial, Acceptance date, group A date, group A short, then similar for group B, C and D, then destination, destination date and engine serials. On this page the destination date is used for the block number, group A is used as country and priority, group B is empty, group C repeats the acceptance date, leaving group D as the probable continuation of group II. All the short columns are empty.
Serial 42-7516 Accepted on 28-Aug-43 Priority 41 Group D Date 31-Aug-43
Serial 42-7571 Accepted on 20-Aug-43 Priority 39 Group D Date 21-Aug-43
Serial 42-7518 Accepted on 23-Aug-43 Priority 39 Group D Date 24-Aug-43
Serial 42-7519 Accepted on 16-Aug-43 Priority 36 Group D Date 19-Aug-43
Serial 42-7520 Accepted on 16-Aug-43 Priority 39 Group D Date 19-Aug-43
Serial 42-7521 Accepted on 31-Jul-43 Priority 34 Group D Date 5-Aug-43
Serial 42-7522 Accepted on 31-Jul-43 Priority 36 Group D Date 10-Aug-43
Serial 42-7523 Accepted on 17-Aug-43 Priority 39 Group D Date 19-Aug-43
Serial 42-7524 Accepted on 31-Jul-43 Priority 36 Group D Date 5-Aug-43
Serial 42-7525 Accepted on 9-Sep-43 Priority 41 Group D Date 9-Sep-43