I also have a 72 Bow Front with an overflow. I love having the sump. It does outgas more CO2, so you'll have to increase your CO2 injection rate which will cost you more (I think I pay somewhere south of $5 a month in CO2 to keep my tank at 30 ppm). The sump will also oxygenate the water, but that is also a good thing if you have any fish. Oxygen and CO2 levels are not inversely proportional, so you can have high oxygen and CO2 at the same time.
I also like having the sump for dosing ferts, Excel, meds or whatever. Additionally with the sump, I tee'd off the return so I have two jets, one on each side of the overflow. This greatly increases my circulation within the tank.
For the sump itself, I used a 10 gallon tank and siliconed some acrylic baffels into it. Then insert the media of your choice and you have your 2nd filter. It's also more convenient than breaking open the Eheim for when I want to add temporary filter media like charcoal or whatever.
My .02 worth...