Roinik That's overkill for a B21A.
I disagree. A DGV is a better carb, particularly if you aren’t a SU guru - they hold their tune better, are easier to get bits for, and easier to understand. I would also argue that that they work better overall.
Those DGV kits are expensive and I don’t know if I would be able to justify the cost if I was still rocking a carby 240, but if there were cheap kits in wreckers, I would replace the SU/Stromberg with one every time.