This isn't much technical help, just testamonial. If you want to go rock crawlin' you won't regret making the swap to D300. I just bought an '81 CJ-8 with a 360 V-8 EFI T-18 and D300. Granny gear is 6:32. The first time I went rock crawlin' I was in heaven. The Jeep would go anywhere. I never had as much fun wheelin' I'm going out again in about two weeks. I can't wait. Oh, did I mention ARB lockers front and rear? They helped some, too. Good luck and rocky trails to ya.
1981 CJ-8 SOA 360 V-8 EFI