Hi, I wanted to give the conclusion to my issue to help the next person trying to do something similar.

The challenge was that I had a Sekine bike and I wanted to change from a cotter pin crank with a loose bearing BB to a sealed BB. The problem was that the bottom bracket was American thread but with a 70 mm length which is unusual.

The solution: I bought a 73 mm Shimano bottom bracket and fit it with spacers. A 2.5mm on the non-drive side and a 1 mm on the drive side. Really simple, well made and cheap.