Whatever bb or bb spindle you use, it should match the crank. I'm sure you can find a spindle that will work with the existing cups, or you can get a whole sealed unit. A standard English thread unit should be fine, but we can't tell you what length you'll need; that will depend on the crank.
OK, cheers for that. Sounds like just buy a complete sealed unit is best. I have these cranks on order'
Vintage - Silstar Alloy Double Chainset 52 / 42 T , Cranks 170 mm, 80's Dawes
From a 1983 Dawes
Square Taper FIt
Chainwheels are 52 and 42 teeth
Crankarms 170 mm
How do I know which spindle is compatible with these cranks?