[72BlckButy]

In my opinion...

If you are wanting to run a larger dia. wheel (20" as you mentioned), I don't think you will be able to go any lower than a 4.5/6" drop without running into issues with turning radius up front. This will allow adequate room for turning and also allow you to retain the inner fenders and no major modification. You can run a bolt in c-notch if you need the extra room in the rear but it is not required.

Now if you would want to go all out there's z-ing a frame but it doesn't sound like you want to go that far into the project. And as far as I know changing to an LS motor will not affect how low you can go. If you have the money, upgrading the brakes would be nice, but not required. As long as the brake system is fully operational, you shouldn't run into any issues with running a stock brake setup with a larger dia. wheel/tire combo.

One thing you will need to take into consideration is the 6 lug to 5 lug swap and also the rearend difference as well. IIRC the 67-70 (early) models had a narrower rearend than the 70 (late)-72 rearends. This will affect your backspacing, so if it were me, I would have the suspension set and then measure for the wheel setup.

Just my $.02... but I hope this helps.