Rear backplane of 12X3,5 chassis does not support NVMe drives. It only support SAS/SATA drives. You can refer below link on supported configurations for various chassis [URL]  [URL] 
R740XD 321-BCPW: 12x3.5" HDD front, 4 x 3.5" mid drive tray and 4x2.5" rear drive cage 400-BDIM : 400GB SSD SAS ISE Write Intensive 12Gbps 512e 2.5in Flex Bay Drive, PM5-M, 10 DWPD, There are currently 4x (400-BDIM) occupying the 4x2.5" rear drive cage bays. The front and mid bays are fully occupied by 12+4 8TB NLSAS drives. We need larger capacity SSD drives in the rear drive cage. The current proposal is to upgrade 3of4 SSD's to equivalent SSD drives at 1.92TB capacity each. My question concerns whether or not it is possible/feasible to transition to NVMe drives in the rear, at or near the same overall cost range of the current proposal?
740XD - Options for upgrading rear drive bay SSD's on 321-BCPW chassis