If the headlights are not lining up with the fender extensions, it's very likely you have a 70 core support.
Your options are to finmd a 71 core support, but 1970 front end parts (extensions, grill, bumper) or try to modify the core support you have to fit 1971 parts. In my opinion, I probably wouldn't try modifying the core support.
As for the different types of core supports (within the same odel year) out there, they had one for a small rad (not very common) and a larger rad. So, in 1970 there are 2 types and in 71 there are 2 types. I think 72 they all had the same support which is the same as the larger rad support for 71.
The reason for the differene between the 1970 support and the 71/72 support is that they decided to move the headlights outwards a little bit for 71. The headlight was now located directly below one of the bolts that holds down the fender so they had to move it up about 1/2 inch higher. The area where the fender bolts to the core support is flat in 1970 but in 71/72 they had to create a "step" for the fender bolt above the headlight.
1970 Misty Turquoise Monte Carlo - 102,000 miles
1971 Cranberry Red SS454 Monte Carlo - 98,000 miles
2014 FGMCC WESTERN MEET INFO: click here