I am about to start a project on my jimny JA 11 swb soft top suzuki. It will be extension of the chassis/ body from the center to about 15" and only the body from the rear to about 6". What i want to know is that "if top speed is not the requirement", should i go for it's stock 660cc epi intercooler turbo engine or the 1000cc carborated will work. Just i need is the low end performance with 31" tyres. Also if someone feels that both these are under powered, than give reasons for the 1300cc.
Regards
Noman

Noman,
I would say that the majority of the members of this board are located the USA.  With that being stated, neither the 660cc or 1000cc engines were ever officially offered in the USA.  (We only received two variants of the 1300cc engine.)  Therefore, you will not likely receive experienced observations or insights on those two smaller engines. 

The 1300cc engine struggles to pull 31" tires, so I think that anything smaller would be incredibly slow.  However, if you will also be running the smaller, narrow axles associated with the 660cc engine, it might be wise to stick with the smaller engine too.

Thanks Jonny.
Very nicely explained. Basically i am living in Pakistan and fortunately, here we have all the engines available. The 1300cc is very expensive as compared to the 660 or the 1000cc. The intercooler turbo 660cc has more power in terms of  accelleration than the 1000cc carborated one but bothers as it needs timely gear changing. On the other hand, the 1000cc moves steadily but smoothly. The diffs we have with the 1000cc is 9:37 only. And with the 660cc, we have 8:41,  8:43 and the 11:45 ones. I am running 11:45 (4.1 ratio) in my jimny JA11 which accelerates amazingly fine in low gears but the top speed is not more than 110km/ hr. Remember i am running 28.5" tall and 6" wide tyres currently. So i have an option to go for 30" with the low ratio diffs ie either 8:41 (5.125 ratio) or even 8:43 (5.375 ratio) which might be able to reach 100km/ hr speed even with 30" or 31". What do you say. Remember my priority is low end power.

As you stated there is a trade off between low end gearing and top end speed with the different diff gears.  I think that the 5.12 ring and pinion along with a 4:1 t-case (1.58 high range) will be very low even with the 31" tires.  I think that the low end will be good, but I do not think you will be able to drive at 100 Kmph (62 mph) for a sustained period of time.

0.795 fifth gear x 1.58 high range (4:1 transfer) x 5.12 ring & pinion x 31" tires (30.5" true diameter) --> 4400 RPMS at 100Kmph

Thanks Jonny again for the detailed reply. I am also considering the 1300cc version. Still thinking. So far the process of frame extension and body work is under consideration. I am planning to start this work in 10 days time. Will keep you posted about the developments.
Regards