I already have a DD 3600 pounder and was wondering if anything else gives any trouble from abuse.

 

You can get a new shaft for them at are a little tougher.

 

from who and for how many arms/legs?

 

here is one place

http://www.enterpriseengine.com/

 

Sloverado,
Really the part EEP markets is just a replacement piece that replaces the lost/broken off piece when you snap the main at the rear of the tranny.

It does not make the trans tougher or weaker since the next time you break you don't break in the same place but up further. So really its just a band-aid to get you back on the road.

A new main shaft would do the same thing as a "HD mainshaft" upgrade.

To the original poster:

Just keep the NV5600 in clean lube and overfill by a quart and it should last you a long time.
The DD's usually make things a little tougher to shift but a good set of hydro's form Southbend and improved driving techniques and you'll be fine.