My first Chevy truck, need some guidance

[Nygaard]

Hello! First post and first time Chevy-owner here

Just bought my first truck - a 1976 C10 step-side. This is a completely new world to me (only wrenched on European cars until now), so I’m hoping for some tips and tricks from the more hardened truck owners. I have already been lurking around the boards for a couple of months and found some good info, but I still have a bunch of stuff to figure out.

First off, the truck and what I will be using it for
It’s a 1976 C10 350 Cheyenne. 96000 miles on the clock and it looks like it has been taken decent care of. It runs and stops as it should, with the exception of a leaky and noisy transmission (which I’ll address later). It's imported from CA, so there’s also a smog pump installed (that will have to go!).
My plan is to use it as my daily driver year round, which means in the 5-90 ºF temp range. 98% of the time on asphalt roads. Only light loads (no more than a couple of dirt bikes + gear).

What I want to accomplish with this project
A solid and reliable build that starts and runs every day without any fuss.
Good mileage (or at least not worse than it has to be ).
Decent comfort and handling.
~300 hp (more = better, but driveability and reliability comes first).
Broad power/torque band.

What I need to decide on at this point of the project
- Transmission: As mentioned, it needs attention.. It’s a regular three speed automatic, so I’m considering tossing it and replacing it with a new/overhauled transmission with overdrive. Which one should I go for, and what (if any) complications are to be expected?
- Engine: A solid engine build - without going overboard. Top end kit (Edelbrock, Summit Racing, etc.), carb, headers, ignition. I really need some help with this one! Should I go for one of the matched kits or are there better options out there? I’ve seen a lot of positive comments on the Vortec heads, but I’m nowhere familiar enough with the 350 to match up a complete and balanced build, part for part.
- Suspension/wheels: What size wheels and tires should I go for, with road use in mind? I want to keep it relatively original looking, but bumping them up to 16” wouldn’t hurt. I’m considering a mild lowering (with new dampers) as well, to improve handling and ride comfort. Suggestions?

And finally, a photo from the day she rolled out of the shipping container - including original Californian dust!