New Guy w/ first C10. Could use some help.

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This is the first C10 I have ever owned. Bought it yesterday after low key searching and dreaming for a few years. It is a 1964 short wheel base. PO(s) installed earlier 283 (with a bad timing cover leak) and powerglide at some point. I suspect that it was originally an I6 truck. I know it was originally manual as well due to the limp clutch pedal still hanging there.

My goal for the truck is a "resto-mod" type truck, retaining most originality but with a modernized drivetrain. I am thinking of swapping the 283 for a stouter 350 and thinking either a Muncie 4spd or 700R4 for transmission. The 700R4 would likely be cheaper and the overdrive would be nice, but the muncie seems to have a higher cool factor. Rearend is totally stock and leaking like a sieve.

Here's what I think I can tell isn't orginal with my nearly no knowledge of these trucks:
-wheels/tire (I plan to find original wheels if I can. Have any?)
-missing side trim?
-wood ashtray door and gauge treatment
-wrong interior color
-window tinting
-Heater control panel (?)
-Seat upholstery
-steering wheel (?)
-Stereo (duh)

I could use some help spotting the other stuff that the POs have changed over the years. The "custom" sail panel trim seems suspect to me. I'm sure that other stuff sticks out like a sore thumb to the experts. Please let me know what you see.

Another concern that I have is the engine install. It looks to me like the engine is sitting too low, the bottom of the fan is below the radiator and the fan doesn't even enter the shroud. Was there different radiators for V8 and I6? If so, would a radiator swap resolve this issue. Are the engine mounts wrong making the engine sit lower in the compartment? I'd like to resolve this issue for proper cooling and fan placement.

Another issue, the heater doesn't work. I think the heater is right for the truck, but I think the control panel is wrong. I saw one non-connected linkage, I'm guessing there are more, but I haven't had time to look. Is there an online resource that can help me with figuring out what is wrong and right?

The truck was an arizona truck, so it is quite solid except for what hacking the POs have done. I think I got quite lucky finding it here in ohio.

Thanks for any help you can provide. Here are some pictures.