Modern Drivetrain Swap into '65 Valiant

Best to copy since pioneering a path is complicated engineering with many dead-ends. The slant-six just barely fit. They offset the water pump housing and cast it into the block, but even then no room for a clutch-fan in your 1965, and slanting the engine allowed a lower hood. A small-block gives more room to the radiator and isn't too tight on the sides. On the plus side, your K-member is the same for a small-block so just need the motor brackets. You also need a V-8 steering cross-link to clear the oil pan, though some 1966 slant ones have the same ~1" drop (slant is ~0.5" drop). You need a 1964-65 oil pan on the small-block w/ a "dent" to not bump the steering linkage. That will fit any 273, 318, 340, or 5.2L Magnum, but not a 360 or 5.9L (larger rear hump). Some people dent a later oil pan to work. I hope to fit a V-8 Magnum intake on my 1965 273 someday, and looks like the air plenum will just barely clear the hood. Some have fit Magnum engines and transmissions, so "should fit", but you must reform the transmission hump to clear their 4-spd transmissions.

I suspect that hood height will limit you to a pushrod engine. Some have fit a GM LS V-8 and transmission, which are common and cheap, with many aftermarket parts. But, insure an early A since the engine bays are ~2" narrower. If you want to pioneer, the Chrysler 3.8L V-6 is pushrod and was used a few years in Jeeps so there is a RWD version (different mounting bosses) and a RWD transmission, hopefully in a 2WD version (no transfer case). But, it has a 60 deg bank angle, instead of the 90 deg of the 3.9L Magnum, which is better balanced for a V-6 but also a bit taller. Most 4 cyl engines that come to mind are overhead camshaft, so likely too tall. When I took the head off my 2.4L DOHC Plymouth engine, I was amazed how far down the top of the block sat in the engine bay. I think most modern engines are fairly easy to wire since the whole harness and controller can be used, with just a few wires to connect to the car, assuming you don't go so new as to have an electronic throttle.

If your main goal is a better transmission, there was a late 1980's version of the 904, termed 999 (I recall), but google. It at least had a lock-up torque converter, if not an overdrive. I saw one (I think) in the junkyard on a ~1980's van w/ slant-six, painted grey so likely US Navy (seems they preferred Chrysler's). The transmission looked almost new, so was tempted to pick it. Note, that the crankshaft hole in a 1965 engine is (usually) too small for the torque converter snout of a later transmission (1967+ recall). One fix is to swap the whole front guts (input shaft) of the transmission to use an older torque converter (lose lockup feature) or buy a custom torque converter. If going the other way, you can buy a thin ring to fit them tight.