The 70 Challenger grill will readily bolt in to the 1971 fender. The headlight buckets are welding in to 70-71 fenders. You will also need the 1970 head light bezels.It is an easy switch. You can also get a hardware kit to get the c clips and hardware if needed.
I did this years ago on my 71 Challenger vert; I ended up going back to the correct grill later. Today, the 71 grills are not reproduced and are expensive when an owner needs one.
Just curious; are you switching because you like the 70 grill better, or that you lack 71 grill, which is more difficult to find and in good condition?