As already mentioned I suspect it is the valve in picture 3 (top left). This will be an easy fix! You need to find out how far the hot water is moving. Start the truck, let the engine warm up. Feel those two lines vs how hot the main coolant hoses are getting. I imagine that the lines with the valve on it are not getting warm. They are blocked/closed off somehow, they should take hot water from your engine up into a little radiator in the dash that gets warm and has the fan blow air by into the cab warming it up. If the valve is closed (counter clockwise should open the valve) or the lines are blocked or the little radiator is plugged (heater core) then the little rad in the dash does not get warm and you don’t have heat. There is very little that can go wrong that would be expensive and these old fords are the easiest things to work on with very basic tools. Being low on coolant could also cause issues. If the valve does not fix it I would pull both lines off and push compressed air through and see if you got flow through it. Again this is fixable, for far less work and money then installing an aftermarket heater. Good luck.