1) Regearing the t-case would be difficult. To the best of my knowledge there are no gears for it. If you do have that much of a concern with super low gears I'd say run some sort of a t-case doubler or atlas case like they use on rock crawlers.... stupid expensive though. I think the 1345 is a slightly lower ratio than the 1356 too, (might be backwards) so you could do a bit that way. **But**, the stock ratio is more than enough for most anything, especially with the diesel torque. Particularity with 2wd - most times I can spin the rear wheels backing a trailer uphill before I stall the engine, and if you walk along in idle it's nice and slow.
2) The 1345 type shifter goes through the removable pan, the later style (87+) goes through a separate opening. Either will fit on the ZF5, and either I think will work with either t-case. The 1345 does shift a bit more awkwardly with the higher floor pan used on a ZF swap though. If I was going to pick one all things being equal I would use the later style shifter, but it's not that much of a difference.