Trigonometry, son, trigonometry! I laced up a 40-hole SA hub to a 36 hole rim, and it took 4 sets of 9 different lengths of spoke (2x, 406 rim, no dish). They were 1 mm different in length. A 32 hole rim will be easier with 8 sets of 4 different lengths. With the drum brake you may need different lengths on each side, so take that into consideration.
The process is:
Draw it all out on a piece of paper. Make sure the missing holes on the opposite flanges are exactly divided from each other. Calculate the length of each spoke as if the wheel were flat. Adjust the length of the spoke by the offset distance of the hub flange.