Does the wider band rub on anything that'd jam it up? If not I doubt it's the problem. I used a wider band in my 904 without any issue. The only thing I can think of is what A/J said about possibly putting the rear sprague together wrong. That would explain the sluggish drive in D position but when you pull it down to manual low the rear band holds the drum. The first thing I'd do before disassembly is an air pressure test. Then I'd check the rear sprague. I attached a PDF file that shows where you inject 30-35 psi of air to test each component. It's for a 904/727 but the clutch ports and band ports are the same