2.7 wpg. What plants ?

I can only tell you about the ones I've tried myself under similar lighting.

Amaranth redroot - Yes. Great plant.
Corkscrew Vals - Yep but watch it. Tends to get weedy and invasive.
Ludwigia Repens - Another great plant. Nice colors.
Dwarf Sagittaria - Grows bushy under good lighting. Runners need to be controlled.
Mayaca fluviatilis - Keep stems out of the shade. Fluff them up often to remove trapped debris.
Brown Wendtii Crypt - Everyone loves a crypt. Nuff said.
Vallisneria spiralis - Same as corkscrew. Spreads fast.

Low light plants tend to do very well under high light. I have some java fern hitting the surface in my 75g. That's under 6*54w of HO T5 light. Crypts grow very bushy for me also. I found anubias prefers some kind of shade so I keep it close to my tiger lotus. By the way, lotus would also grow great in your tank. Rotala indica is also quite easy and looks good. I would experiment lots. Most plants will do well under 3wpg but you'll find DIY CO2 difficult to maintain on a 70g tank. Steady CO2 is not a luxury but really is one of the keys to preventing algae.

You should be able to grow almost anything with 2.7 wpg in a tank that size. I think thats a better light setup than 3.5wpg because that would require alot more work triming all the plants. Most low light plants do fine in high light from my experience, Trim back the crypt and it will probably grow closer to the ground and nice thick leaves.

Your getting pressurized CO2 right?

Ditto what has been said, you shouldn't have too many limitations...But...I would 2nd the motion for pressurized CO2. DIY will work, but pressurized will be worth it in the long run.

From my experiance with a DIY system on my 55g (compaired to a pressurized system), The DIY is alot more tedious as I had 3 2L bottles running at all times and the CO2 was dispursed in bursts because the DIY system had to build up pressure to open the check valve and then lost pressure and the check valve closed. I wasn't able to maintain high enough CO2 levels.

Now with my pressurized system I get a constant amount of CO2 as there is enough pressure in the system to keep a steady flow rate. I'm also able to get a high enough CO2 level (25-30ppm) and a CO2 tank (depending on size) can last anywhere from 6 months to two years.

The main cost with a pressurized CO2 system is the initial purchase of CO2 tank and regulator/needle valve/bubble counter. The cost of CO2 is cheap. $32 CAD to fill my 20LB tank which should last me over 1 year. The cost of tanks and regulators vary depending on the quality (regulator) and size (tank). All CO2 regulators are not the same. They vary in quality depending on the manufacture. For example I purchased the Milwaukee Co2 MA957 Regulator Set for $80 US it lasted 2 days before the solenoid burned out. While waiting to warrenty the regulator I purchased the
AQUARIUMPLANTS.com's Co2 Regulator for $140 US. Not only is it still working to this day, it holds a constant pressure, is easier to read, and is easy to adjust. As for CO2 tanks they come in various sizes (2.5LB, 5LB, 10LB, 20LB, 50LB) they also come in two different types. Aluminum and Steel. The Aluminum has to be recertified every 5 years and the Steel every 10 years (atleast thats the way it is here in Canada. Should be the same in the US). As for choosing the size of tank I go with the largest one that will fit in the space I have for it (although the 50LB was too expensive). The difference in filling the tanks is only a couple of dollars so it makes more sense to get the largest one and fill it less often.

As for using paintball CO2 tank I belive those would be way to small for a tank your size and you would be constantly changing them. Plus from what I've seen the regulators are more expensive than the regular CO2 regulators.

If you decide to go with a pressurezed CO2 system I'd highly recommend checking out the CO2 equipment offered at aquariumplants.com and more specificly their brand regulator. No point bying the cheaper one and having it break on you (trust me I've been there).

I hope you find this informative and helpful.

Regards,
Jason

Just to add to that...

Yes, there is a quality difference. You can spend a couple hundred on a regulator or <$50. I bought one from Harbor Freight for ~$30 and it worked for 2yrs. A friend gave me an OLD regulator he use to use for beer making and the thing is rock solid even though it looks like dirt. I also have a Milwaukee 957 which was a little inconsistant initially, but I disassembled and cleaned the needle valve and it's been great for several months now.

Ask around. Ask friend/relatives, you never know who is/was into beer brewing and may have a spare regulator and/or tank laying around. Some get into it with a 5 lb tank to start then move on to bigger ones and don't use the smaller tank anymore. Someone works at a auto shop? Anything that may have some welding associated with it could be a great place to score some stuff Even if the tank isn't certified, most places will give you credit if it is in good shape. Do look for a facility that exchanges the tanks, that way you won't have to worry about paying for the tank testing every so many ways. Welding supply companies, beer making supply co's, even beverage co's may do this. If the local convenient store sells kegs, they may too!
I agree too that the paintball set-up may be too small for you.
You have read the CO2 injection sticky at the top of the main page haven't you?