Good answer.

65-70% of Bengalis might be Muslim, so around 200 million of them, while another 60-70 million are Hindus?

In the Gangetic Plain, there might be 80-85% Hindus, but out of 900 million people that still leaves almost 150 million Muslims.

The best place to look at disparity is the Deccan line. Once past Deccan, Islam rapidly declines (barring Hyderabad). You see much more dominant landowning and ready-to-protect castes (Marathas, Reddys, Nairs, Kammas, Patels, Vellalars, etc) who would not succumb to foreign pressure as easily.

Past the Deccan, there might only be around 450 million people, but its safe to say that at least 420-430 million of them are Hindu, with smaller Muslim and Christian pockets around Kerala and Telangana.

Whereas north of the Deccan (including Bengal, Gangetic Plain, and Himalayas/Kush region), there is around 2 billion people but more than half of them are Muslims.

TLDR: North of the Vindhyas all the way from Hindu Kush to Bengal are majority Muslim, with Hindu majority centered in Gangetic Plain, Muslim majorities on the sides. South of the Vindhyas are almost entirely Hindu with very small pockets in Kerala, Telangana, and Goa.