from what I know, the 3000 is an SAE2 trans and has the same offset as an MT-643.. you would need the same flexplate as an MT643.. although im going from swapping out an AT545.. my DT360 book is too old to show the 3000s.. but from what I understand the 3000s were designed to fit in place of an MT-643..
your flexplate will be the same as a DT466.. they are equals..
this is the flexplate i used to swap my AT545 out for an MT643.
Navistar part 467037C1. in my case the spacer got removed altogether and the flexplate fit right on the flywheel then I put the "washer ring" on top of the flexplate, used shorter bolts..
again im not familiar with going from a spicer.. just an AT545..
be careful not to buy a flexplate for a DT-466E (which is where most of the 3000s ended up_.. the DTA360/ DT466 mechanicals have an 8 bolt crankshaft hub.. the DT-466E has a 12 bolt..