The ASI is in IAS.
CAS is a necessary step for some calculations, obviously - and it's a requirement to provide a correction in the manual for part 23 aeroplanes, which includes your Beech.
There was a phase in the 50s and 60s when some American manufacturers provided limits and speeds in CAS, not IAS. This was a really stupid idea and was eventually beaten out of them. If your manual does that, make the conversions using the table in the POH performance section, and fly to IAS.
G