Thanks. No choice but 3.50 if I want to stay with K70's or any other recommendations?
Even the 3.50x19 K70 is a beefy-looking tire and would definitely suffice for your application in the absence of the 4.00x19. However, a good substitute 4.00x19 for our vintage bikes is a Duro brand HF-308 4.00x19 and they are readily available. They are a copy of the Pirelli MT-53 tires. Several vendors carry them in other sizes as well.
Saw those but had to change my underwear after seeing the prices! Plus they don't say how old they are. According to online sources, the industry standard to replace is around 6 years even if they look good.