having done a L series conversion, modifying x-member, mounts, and brackets I can tell you that a bolt in x-member swap is definitely the way to go! Especially if you think its such a PITA just to have to trim a little bit off the castor rod mounting brackets and gearbox x-member - Forget doing an actual conversion!

I remember the problems you guys had getting the 180B x-member in your 120Y but I can't for the life of me figure out what you did wrong!

don't be put off Tom - here a few photos i took when I was doing my 120Y L series swap. Have a look and make up your own mind. I paid about $40 for all the 180B stuff I used in the conversion anyway. I didn't have any problems...

mods needed to allow fitment of 180B engine x-member
comparison of 120Y & 180B x-members
closer look at the x-members

the sump is only an issue if you want to fit a L20B or late (Stanza) L16, which both use the same forward sump. The 180B's L18 sump obviously fits if you use the 180B x-member. So does the 1600's L16.

Best box to use is one of the short 5spds - Stanza or R30 Skyline. The Skylines is heavier and stronger but has a big gap between 2nd and 3rd ratios. The Stanza box is strong enough and has better ratios, but it also has the quirky reverse pattern:
R 2 4
1 3 5

there's some old threads around here where this was discussed in some detail.