i solved all of the craked manifold issues on my 64 by installing a late 70's early 80s granada/fairmont head, you have to modify the intake slightly by drilling & tapping new holes for the watertube plate to bolt onto & i can't remember which pushrods you end up using (it's been10 yrs since i did the install on mine) but you use the the adjustible rockers from the old head. works increadibly well this conversion. you also get the benifit of way better cooling, unleaded compatible valve seats, and bigger valves which of coarse boosts yer horsepower up a bit.
have over 20,000 troule free mile on mine since the conversion.

far as the corner windows go.....
bigg'nuff job to change em over & do it right. it would involve drilling out the spotwelds and removing the entire section. my personal opinion on just cutting out the window opening section and sticking it in would just be a huge butcher job that likely wouldn't last because of the flexing of the truck body filler would likely end up eventually cracking.
mikie
near ottawa canada
64 econo p/u 58 merc 100 panel