The 146036 would be a bad choice in my opinion. Passenger side is the preferred location.
Look at bracket Part #: 15249-VCB. They don't have a picture and the description is messed up.
https://www.vintageair.com/instructi.../15249-VCB.pdf
It says it is for a 250 and won't fit with HEI. The 292 is 1.75" taller, which gives more room. I don't know if the 1975 6 cyl HEI with External coil will fit. Myself, I would give it a try and modify the bracket if needed.
I made my own bracket, which is similar, so I can't say if the Vin Air bracket will or not.
Their kits includes a plate that covers up the holes from the old heater.
I've helped install a couple of Vin Air, but they weren't on trucks.
When you use their kits in conjunction with original factory controllers, either AC or heat only, they provide you with linear, slide resistors that you have to attach to the control arms on the back of your original controller.
Really hokey setup. I mean really hokey.
I would op for one of their control heads. Costly and has their name on the face, but a cleaner looking setup.