I have driven a S model LJ for over 100,000 miles and can tell you if you go to the trouble of getting a 186S WW Stomberg and not a 253 Stomberg, if used in a moderately enhanced engine you will be able to attain speeds well in excess of the speed limit in any state ( 180kmh ) and if driven sensibly can get up to 30 miles per gallon on the highway. By no means a slug!
I have driven one XU-1 that did very much dissapoint me with its poor performance ( it all about how you set them up )
A big cam is not the only answer.
A big cam and a large four barrel may mean good top end, but can also mean no bottom end performance.
I also have a GTR which has moderate internal changes with a stock can and can easily and quickly reach 160 kmh.
in this case a balance, large valves, slightly higher compression and a HJ inlet manafold, 1 9/ 32 inch venturi single carbie and compact header type extractors is all that is reguired. ( It is no slug )
I consider that GTR's are getting much rarer than XU-1's and fakers, I would not consider changing mine any more, and will probably remove the extractors some day to make it more original.
Please keep your original carbie and manafold as they may be a bit different to the stock ones anyway as my inlet manafold has an X cast into it, which is not present on any other stock manafold I have seen ( and I have seen lots )
Mike.