IDK what a T600 is, but the ZF S5-42 is a strong 5-sp OD w/granny 1st. The T18/19 has a granny, but it's only a 4sp, so no OD. The NP435 has one of the deepest granny gears ever, and it's so strong that it was used for MANY years in 2-5 ton trucks. Some big UPS vans still use them.
.
So you have plenty of options, and not all of them are expensive. A decent NP435 can be had at a junkyard for under $200 if you shop around. Maybe even less for a 2WD. I've seen several T18/19 2WDs just recently, and I have a couple of spare NP435s that I'm planning on rebuilding soon.
The TOD is one to avoid, but they're so rare that you'll probably never see one.
