Pete - For starters, check out this
1957 Chevrolet Accessory Installation Reference Manual. Page 108 is where the Truck Deluxe heater instructions begin. Even thought it's for 1957, it should still cover your 1958.
It doesn't specify the hose sizes in there, but a 1929-57 Chevy Master Parts book calls for both 5/8" and 3/4" ID hoses, cut from bulk stock. You should be able to measure the differences in the heater core tubes to match hose to tube sizes.
On the temperature control valve, temps are controlled by a Ranco valve assembly.
This 10+ year old thread by fellow Bolter hd4unm
Deluxe Heater Assembly and Installation Tutorial on his 1959 Apache should provide you most everything you need concerning what goes where, how, why, etc. He did a pretty outstanding job creating a detailed "pictorial" to help fellow Bolters with this task.
Need more help???
