Okay all j30 are r200 viscous lsd Q45 are r230 they are to big. So this is how i builded one i found a r200 out of a 86 300zx longnose with 3.70 gears just look at the trans speedo gear they are color coded blue. Than i took a j30 r200 viscous shortnose i took it apart and remove the inner hub and bearing and races take the ring gear off it want help you. Now take the 300zx r200 diff cover off remove the inner hub dont mix any parts the shim that is on the left must go back on the left and the one thats on the right must go back on the right. Now remove the ring gear they both have 12mm bolts. Now install the 300zx ring gear on the j30 viscous hub clean and use locktite. Put the j30 hub inside the 300zx housing with the j30 races now put the shim that was on the left side back and the one on the right back install everthing back and tighten the back lash should be the same as it was on the 300zx before you took it apart. r200 is finshed dont forget oil. now you can use the driver side r200 alxe from a 280z the splines are the same. here is the trickey part you need the pasenger axle from the j30 its longer and must be used or you want have pos track. What i did is cut the five bolt flange off and my friend finshed it down with a lath than i took the a r200 axle from a 280z and cut the shaft as close as i could get to the yoke than i finshed it down with a grinder and sander than i put them together in the lath so they would be centered and spot welded them with a mig i did not weld the shafts are they will break. I spot welded all around than i welded in the lath it came out perfect i installed this in my 1971 240z with some other parts from a 280z and drove it today about 80 miles it great i have pictures if somebody needs to see them but i dont know how to put them on this site im new. but if somebody needs them you can email me at email@example.com and i will be able to send them to you i hope this can help somebody it wasnt hard.
Lots of information here, but it is very hard to read due to the spelling, grammar and run on sentences. Looks like there is good info here, but just hard to make out.
Correction though. The Q45 differential is NOT an R230 like the twin turbo differential. It has a 200mm ring gear and will fit inside the R200 case from an 88 300zx.. I have personally done it with pictures to prove it. However the splines ARE larger than the z31 series and the s130 series differentials. Along with almost all other R200's for that matter.