Nice car. Wow, the 260 is super rare and was somehow even worse than the 350 diesel. The block should be plenty strong but the tiny 3.5 bore makes it useless to build. I say a nice Olds 350 or Olds 403. Both directly swap and for 400 hp, a stroker isn't needed. I say a 68 to 76 Olds 350 block to start. Cutlassefi, Mark Remmel in Florida, worked with Edelbrock to bring a new and improved 68cc aluminum cylinder head with a better chamber and flow that allows a mechanical fuel pump to be used. He also brought a direct fit connecting rod from Molnar, Scat also now has a rod, although not as good but cheaper. He also worked with Mahle to bring a 10cc dish forged 4032 piston with their 1mm ring pack for the Olds 350. They are available in 4.065" hone to fit, up to a 4.155" bore which most Olds 350 pre 77 blocks will take. He can do custom cams, including a roller cam as well as complete builds to easily hit your 400 hp number. He is also a trained aftermarket EFI tech and sells Holley and AEM systems as well. Of course that leaves your potential TH200 or it may also be a TH250, look for a stud near the cooler lines if the pan doesn't say Metric. It is another weak transmission, mine lasted one day behind my mild 403. Also 2 something gearing weak 7.5" rear will need addressed. Good luck and a nice Cutlass, love the Colour.