Yeah...the stock rear-ends for the earlier luvs were pretty low-geared. Putting the weber on will help boost your power a lot...but to get more top end speed out of your truck you will need to replace your rear-end. These are what to look for:
79-80 Luv 4sp and Auto have 3.73 gears
81-82 Luv 4pd have 3.42 gears
and..
81-82 Luv diesel 5spd have 3.73 gears (this 5 speed will not fit your truck because it will only bolt up to a diesel block)
81-82 Luv diesel 4spd have 3.42 gears
The 3.42 gears would be the best...and if you can get your hands on a 5 speed out of a newer Isuzu truck...that will help a lot too.
You can also use one of the ratio calculators in the information section to come up with the right tire sizes to go with your new gear ratio to do exactly what you want.
If you decide to get a 5 speed transmission...make sure it will bolt up to the g180 or the g200 engine, make sure it is out of a shortbed, and get the driveline intact with it...because the front yoke splines that fit any 5 speed are way different than a Luv's and hard as hell to find.
My trans and rear end was out of an '86 Isuzu shortbed and I used the springs from my Luv , the brakes and shocks that came with the donor Isuzu's rear end, and all I had to do was re-locate the little brake line junction box, put in some new u-joints, and do a tiny bit of fabricating for the emergency brake setup and I was right back on the road.
If you want to know more about how to give your truck better power...you might also want to take a look at these 2 threads:
http://forums.luvtruck.com/viewtopic.php?t=10702
and...
http://forums.luvtruck.com/viewtopic.php?t=10636
I hope this all helps you.