@joinami
4M40 would need its own tranny and the package will not be cheap....say a good engine and the manual gb should be around 150k......plus the mods to adjust the gb housing in the gen 1 would be another NOT so favourable factor (lots of cutting and welding to set it up properly is required as it is much larger in size than your existing one).....my advice....NO!
If swap is a must..... a better option is to go for a 6G72 and mate it to your standard gb (though the original on the V6 is different than yours but still compatible.....or buy a half cut V6 with an auto tranny.....150 horses in the swb are good for a quick dial turn to 160 and beyond:))! BUT.....a word of caution....if you are eyeing Northern area touring on a gasoline motor and unless you plan carrying the trip fuel all along......it is NOT a good idea.....trust me as I just replaced a busted fuel pump owing to the poor quality of petrol (which basically is meant for m/cycle carbys there than to be fed to an EFI motor....and god only knows the fate of those injectors I've probably screwed on just three odd trips)!!!
Coming back to your original motor..... addition of a turbo as suggested earlier is the cheapest yet most do-able of the options available....though 160 wont be possible but on good days 120-40 shouldn't be a prob either! Plus overtaking and going high altitude should be more fun! And diesel out there has almost the same cetane rating....to cater for those big trucks plying on those routes!
Do this for the moment I'd recommend.....its both practical and economical..... and if that speed bug resurfaces again sometime later....the V6 3000 option is no major rocket science either!