I've got a 1984 Dodge D150 with a 318 in it and I was curious what would be the best most affordable rout to go to squeeze some HP out of this engine. What I had in mind is to put a sock 340 cam in it with a four barrel intake/carb. What kind of heads should I run on it? Does what I have going so far a good start, and also how much HP can I get out of it from that I want to be around 300hp. Also if any knows what is the best transmission to run behind this engine?

Welcome to the board! There have been a number of threads on the subject over the past week. Here are a few:

https://board.moparts.org/ubbthreads/show...e=7#Post6938327

https://board.moparts.org/ubbthreads/show...e=0#Post6940560

https://board.moparts.org/ubbthreads/show...e=0#Post6939165

My 318 combo was built using 340 heads w/2.02 valves, stock 360 cam, Edlebrock SP2P intake with a holley spread bore carb (great carb). If was to do it over I would pick a more agressive cam and maybe use small valve heads.
It got used with both Auto and a manual OD 4 spd trans, Iliked it best in my short wheel base 1/2 ton with th manual od and a few degrees advance on the cam timing

If it was me, I would get a set of magnum heads for that 318. The thing is, you could probably call up LKQ and pick up a 360 magnum that could work with the proper flexplate. Already has the magnum heads and more inches too. Get a magnum style intake manifold and use your existing distributor and away you go. You'll have a hydraulic roller setup too. Cams are a little more expensive, but if the motor is good, you can reuse the lifters.