a 20's lift , if in good condition and factory set relief should get 800# or so off the ground and the front end will be reallight. an exceptional lift might get 1000# up at high rpm and lotsa front weight.. but figure 800 as a realistic number. that's about a 6' dual ganged disc, again.. depending on construction. My 2n and 8n will lift a 6.5' box fram ford dual ganged disc to full height, lift groaning.. i suspect it's around 750-900#.. and is fundamentally the same lift as that fergy 20..
soundguy