sounds like a plan. is the toyota track width close to what you need? park brake be an easy hook up? drum rear and disc front brakes? are you closing the door on the jag undercarriage swap or just weighing options?
grab the master cylinder, booster, even if it is too big diameter, prop valve and brake rubber hoses from the same vehicle as well. might have to custom fab a brake line or two to go from old chevy to newer toyota. thats pretty easy though. the rubber lines may not fit for your application but would have the proper connections for the calipers etc so you have the correct fittings in hand when you go for parts. if you can fab a new frame mount for the toyota hoses and adapt from chevy to toyota on the frame it would be simpler if you ever need a repair. when grabbing parts also grab the driveshaft so you have the correct end for the machine shop guy who adapts your driveshaft to go from chevy to toyota. if the length is close, like the original shaft ends up being too long, the shaft could be cut off at the welded on yoke for the original joint, then shortened accordingly to fit, and the end welded back on. then an adapter u joint could be used that fits the chevy shaft and the toyota pinion yoke. not sure of parts access where you are though so it could be better to simply rig the shaft with the correct u joint for the toyota axle. I also recommend starting a "parts log" where you write down part numbers and application for any non original '59 parts. just to make it easier if you need to buy another one some day.
different gear ratio would be an option for sure, instead of a whole driveline swap. keeps the truck more stock. I would keep the old stuff around for a bit until you figure out if you like the new system. there are online calculators that will help you figure out engine rpm at different speeds if you wanted to compare what the difference would be. different sized tires will also make a difference.
http://www.csgnetwork.com/multirpmcalc.html
https://www.crawlpedia.com/rpm_gear_calculator.htm
https://spicerparts.com/calculators/...rpm-calculator
https://tiresize.com/comparison/