I have a '41-'47 GMC(similar to a '39) I converted to 4WD. Used full width, 10 bolt front axle from Blazer, found(rare) full width front fenders. Not so usefull for your application, you could just use Wagoneer or some other (correct width for your '39) axle(match ratios w/ rear axle). While at it, it would be useful to parallel the front springs and put the fixed perch on the front for better ride, not necessary, however. Used a Scout power steering box, mounted to the frame-pittman arm must extend forward of the box, crossover steering, whatever trans/transfer case you wish( I used a 542 and 241D) Have to be careful to get axle knuckles w/ the correct ratio w/ pittman arm, Scout front axle complicated because of long steering arms on knuckle, steering arm has odd ends. Have px.

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.