New video card for Mid 2010 27" with 1 Gb card

[BassLake]

I was recently gifted by my F.I.L. his old Mid 2010 iMac which is still a beautiful machine. He wasn't able to upgrade any apps or MacOS so he bought a new iMac just before Christmas. He wiped this one and I was able to install a fresh copy of High Sierra but can't find many apps that will install with H.S. because now require 10.14 or newer even Pages or Numbers. After watching lots of YouTube, my thought was to change the video card to something that can support Mojave or Catalina and then get a working machine. I purchased a new Nvidea 765 2Gb video card as that seems to be one that will allow for this (its a "Metal" card) but it does not fit onto the heat sink that is in my iMac. The iMac came with the original 256 SSD, 16Gb Memory and Radeon HD 5750 video card w/ 1GB of memory. Both the old card and new card are the same size. The videos I watched show upgrading a 5670 with 512 Mb and that card is a bit smaller is length but a proper fit. The problem is most of the little memory or processing chips are located on different areas of the card and don't match up to the "multi level" surfaces of the heat sink and make for some substantial gaps. The big main processor chip and 4 mounting hole line up perfectly. Has any one faced a similar issue or knows of a suitable replacement card that they can point me towards? I'm brand new to using a Mac computer but have nearly 30 years using IBM machines. Any help or suggestions is appreciated. Thanks in advance.