FVN and Glasstek both have the parts in fiberglass, it wont be fun mounting the headlights and you wont have any crash protection, but its possible to do it. VFN has the one piece fronts for $770 or so. The reproduction front fenders are $500 each and need some definite rework so the marker lights fit decent, the hood will bolt to the fenders the same, but the 70 front ends are very different. Everything on the 70 GTO is that year only in regards to the front end, I think the only thing that will interchange is the inner fender wells.
If you are a skilled body man and fabricator you could recontour the fenders you have to fit the 70 front end, but the amount of work you would have in them would probably be more than buying glass or repops. Original GM 70 GTO fenders bring big money in decent shape, so unless you want to have $5k -$10k in the front end, and you are dead set on doing the front end swap, you will need reproduction stuff.
Look at the fenders you have and look at this one. The difference is where the chrome bumper goes near the marker lights. You have to fill that in and get the body lines right, not a simple thing.
Finding a front bumper in good shape is a pain, I have been looking for 15 years now and I really dont want to go fiberglass for it, at least not on this one.
You do realize the 70 bumper is very heavy and it makes it difficult to get traction with all that weight so far ahead. Mine will roast the tires at 35mph like its standing still and it wouldnt hook decent even with slicks. If you want the look, well its possible. I have two others I plant to build, a T37 and a Sport LeMans, neither will get the GTO front ends even though the T37 is going to get a glass front end to make it light.
I vote for keeping it a LeMans too, but I will post any pics of my 70 you need if you really want to change it. In my opinion its a lot of cost that could go to buying a real GTO. They are still out there and not completely out of their minds in prices. I had to pass on one for $2500 this summer, I really wanted that car too...