how much horsepower from my 318?

On a budget you can change the cam with springs lifters and chain for around $200 and probably get an easy 50 HP, more with the mods you already have. You can get 340 springs from rock auto for around $50 a set. The summit cams are a good for the money, and the 340 cam works well too. It is very easy to add 100 hp with no penalty in MPG if you can resist the temptation to use the extra power all the time.