Min Match for full Ist cousins?

[tempest]

We are trying to confirm the identity of my Grandfather on my maternal side. I am female and my father is dead. However I have been autosomal tested along with a male ist cousin whos mother is a sister to my dad. The test came back as matching with 420 Centimorgans with 51 the longest block.
Is this enough to indicate full ist cousins? Or is there a possibility that the respective ist cousins parents, had different fathers?
Any help would be much appreciated. We are also in the process of getting my brother tested for auto and y DNA but results are not in yet. We hope to get a direct male descendant of the "possible" grandfather as well but this is proving a bit elusive at this time.
Thanks in anticapation.

[ltd-jean-pull]

400 or so shared centiMorgans is about half of what you'd expect for 1st cousins.

[prairielad]

Amount suggest a possible half 1st Cousin
This amount suggests that your father and aunt where only half siblings.

According to Blaine Bettingers chart, first cousins share between 553 to 1225cM, on average 874cM
https://isogg.org/wiki/Autosomal_DNA..._version_3.jpg

[Biblioteque]

My husband and his known 1st cousin share 930 cMs.

My known 1st cousin, 1x Removed and I share 452 cMs.