Springs are not welded in place. The perches should not have to be moved at all unless you are going spring over. You can remove the other leaf to even it out but, springs are pretty cheap to buy or someone going to lift springs many give you some in good shape. Caster won't change unless...
Having a hard time pictureing the linkage, mine is long gone. If you put up a want ad here someone will have the parts to convert it to floor shift. An easy swap to do. Sorry not much help.
I put the smaller jets in mine and it worked out about perfect. I live at about 7000 and go to 12000 with no real problems. I've never had it below 4500 so don't know what it would be like lower. I'm running a stock F engine. I think you should try it and starte with fresh plugs so you...
You need to change the jets in the carb to make it run richer or leaner. The adjustments are only mixture at idle and idle speed. I found the right jets inside my carb as spares but I have a 71, not sure they were in the later ones. The Haynes manual has foot notes about what jets to run at...
One easy thing to do is swap tires front to back. That will atlest tell ya if it's a wheel out of balance. My friend changed U-joints and had his driveline balanced before finding his new BIG O tires were crap.
On my 71 the arms would only take the Toyota Blades. I bought new arms out of the ANSCO catalog from Checker and now can buy the ANSCO blades anywere. The new arms also have more spring to them and do a better job.
I always crack the insert out of the plastic knobs and epoxy them into a hardwood blake, than turn it on the lathe. You could do the same with alu or brass. Also metal knobs suck in the winter were I live.
You can get a plate adapter for your 465 from Classic Crusiers in Salida CO. I have one waiting to go in my 40 but no $$ for drive lines right now. I opened mine and replaced the bearings and bushings- one HD puppy.