as for Diesel gearbox, they have really bad ratios for petrol engine 
Are you sure? Have you checked BMW ratios? 
well regarding these gear ratios for a 320d
1st=1:4.11, 2nd=1:2.25, 3rd=1:1.4, 4th=1:1.0, 5th=1:0.8, 6th=1:0.66 & R=1:3.73
to have something similar to a 5 speed E36 gearbox with a 3,45 diff (without regarding the 6 gear) the diff ratio have to be around 4,27! so to have shorter gear it need 4,45 for example, when 3,73 or 3,91 can be used with E36 gearbox, and 4,45 haren't really easy to find or cheap, especially with an lsd
