Looking for some basic mods?
Good news/Bad news
The bad news first.
You do not have a muscle car.
The 273 is the smallest & most difficult to get power or a power return from.
Your best bet is to get a bigger engine.
The good news;
Much is available for the mini motor!
It will take on almost all the Hi-Po parts from other small block engines.
Any other small block will interchange with it.
Basic upgrades that don't kill the bank are;
Open air cleaner
4bbl. carb (not larger than 600cfm's.)
Intake manifold. (I seriously recommend a dual plane here)
Headers & Exhaust
A good combination would be the Summit racing headers hooked to what they recommend, Hooker headers. IF they have it in 2-1/4, that's what I would do for the small engine. 2-1/2 I'd better suited for the larger engines making good street power.
A camshaft change is also reasonable in cost, but, it is a little advanced and I don't know if your up for the challenge or cost.
My min. Engine size recommendation would be a 318. A 360 would produce a lot more torque for less.