A lot to digest here. The e28 is a sedan and had the M20, B32, B34, and the multivalve motors. I’d never had a B35.
A 635csi is an e24. The 635 only had the B35 in 1989. Check to see if it is actually a B35.
B34 Head:
A good upgrade. Less prone to cracking than very early B28/B30 heads. The...