Pull the plugs on the 350 and put a few squirts of penetrating oil in each cylinder, then turn it over by the balancer center bolt. However, don't get carried away and break anything. If it does not turn over easily, the pistons may be rusted to the cylinders.
If it does turn over freely, then pull the valve covers and get the numbers off the heads and the numbers off the block pad. There are many sources online to look up the numbers and verify what you have.
Its probably a basic 350 with smog heads and low compression, which means about 210-220 HP at best. However, that engine is the starting point for most builds, and it works fine for a daily driver. Adding a new dual plane manifold, good carburetor and headers will bring it up to about 240-250 HP, and swapping in an RV cam will get you another 25-30 HP. At that point you really need better heads and more compression, so further mods get much more expensive.
Bruce