The SU manifold for the B30s is a kinda ugly piece.

The carbs have the same spacing as on the B20 (you can
use a B20 SU heat shield on the B30) so the outside
cylinders have longer runners than the inside cylinders.
Also, the thing is cast iron and has both intake and
exhaust on the same piece so there are heat soak issues.
Also, the exhaust half is prone to cracking.

That said, my 164 with twin strombergs (same mounting
as SUs) was a sweet running little car. Of course, it was
also kinda thirsty, it had too much bondo and a terrible
paint job.

If you got a car that doesn't work, fix what's broken.
If you really want to change things, change as little as
possible or be ready to spend lots of money.
chris