I have the exact 75g tank & stand (i just broke it down so it aint running anymore) - and I used the 20L for a sump. Do NOT cut the midframe - I got my 20L under it by knocking out the 3 boards on one side. they are glued and stapled but will knock out easy. Only tricky part is there are a couple satples that go thru the front & back faces into the #1 & #3 sideboards and the only way to remove these is just cut them by using a large flat blade screwdriver as a chisel and just shear them. Once these staples are cut the boards knock out very easy - just use a little $1.25 bottle elmers wood glue and some 3/4" wood screws to re-assemble the boards wet weiththe glue and the stand will be good as new.
Look for these staples about 18" up from the bottom, and they go thru the front face and into the end sideboard. my stand only has 1 staple or tiny nail on the front & back about 18" up from the bottom. From the front it just looks like a tiny hole on the left or right side on the same face the doors are on....when you find this hole and look at the end - espically with a screwdriver wedged in the end - you'll see the tiny nail running thru the front face into the end boards. Once these are sheared - the 3 end boards just push (with a little effort & a mallet) into the stand. It's best to take a piece of tape and label each one 1,2,3 and which end is (UP) cause they are stapled from the inside, and since they were glued - a little wood may break off with them, but it all glues&screws up after your done,
This was the easiest way I found to get my 20L under the thing, with the least ammount of damage - and I did mine with a full 75g tank on top of it.....Was just REALLY careful, and spent a couple hours on it.
lemme know if you want pictures, i can take &post some of mine