It is hard to say for sure whether it is a variant or not until the DNA tests have been done. However, it sounds close enough that it is fairly likely. Surnames have changed a lot of the past few hundred years and sometimes one generation would spell it different than the ones before or after. Also, immigration officers, birth certificate registrars, and so forth, would often mangle names on official documents.
If you do decide to get tested, FamilyTreeDNA will allow you to compare your results against their entire database, so you can find matches in ANY surname, including other possible variants. Y-Search offers a similar service.
If you haven't already done so, I recommend posting your family tree on our pedigree forum. All pedigrees posted on that page will be transfered to the official surname project website, so that other researchers may find you and contact you:http://www.wfnforum.net/index.php?topic=1975.0
If you want to see the pedigrees and results of other researchers, please go to the project website. You may find other researchers already working on the same family lines that you are:http://www.worldfamilies.net/surnames/f/ferreira
Also, here is a direct link if you want to go ahead and order a DNA test. Keep in mind that even folks that are not direct paternal/maternal decendants of this line can still get tested. To do so, find a relative who IS a direct decendant and sponsor their test for them. We provide comparison for test results for our project members on our surname websites, and you can use FamilyTreeDNA or Y-search to do a global comparison match (over 1/2 million DNA tests for genealogy have been conducted so far):http://www.familytreedna.com/DNAList.asp?Group=Ferreira
Also, please check out our website if you want to learn more about DNA testing and genealogy at:www.worldfamilies.net