First round of issues found on the 63 Valiant

If you remove the oil pan (jack up engine ~3"), I wouldn't be concerned with washing the crud down the insides, using diesel, kerosene, or gasoline (don't smoke). Seems like the worst it could do is sit on the camshaft, but I think you can reach that from underneath with a paper towel. I would force clean oil thru the pressure switch port first (use a hand garden sprayer & rubber hose) to keep any grit out of bearings.

Your compression numbers look good. I hope you got a mini-starter (from Magnum engines). They are half the size and weight, and often cheaper.

The heater core is cheap (~$25), at least for my 64 & 65 w/o factory air. Any slant exhaust manifold will fit. People ask $100 for a used exhaust manifold on ebay, though you can get one attached to a whole slant engine for less. If a smog car (~74+), you need an EGR block-off plate (ebay) or keep the EGR on (no vacuum). Some later engines had a different choke pocket that might not work with your carburetor. It looks like a P.O. did the infamous upgrade of "make engine bay look purty by spraying black paint over rust & grime", as they did on my 2 A's. Time with paint remover, a wire brush, sandpaper, and new paint got it looking nice.