I am not sure of the rear axle its stock posi from factory, thats all I know. The transmission is origional powerglide, but I am looking to go to a m20 or m22 (like I have in my elcamino) four speed but im not sure yet. It has a rochester carb its very small I belive a two barrel. The intake I have not even looked at, Im planning for a dual plane edelbrock unless I find a better suggestion. As for my budget, I dont really have one in mind, I suppose I would like to keep the engine modifications under four thousand because anything over that seems like a waste of money on a 307 and should just swap engines like I have heard advsed often.
I believe anything you do to the engine will be rendered moot by the powerglide transmission. The power glide is a performance killer mated to a low torque producing engine like the 307.
If you want an immediate return on the investment, convert to the m20 and dual exhaust. See where you are. You WILL notice the improvement. You MAY be satisfied.
If you are willing to sacrifice 'originality' with the transmission, the intake manifold, the cam, and the carb; why not the bore and stroke of the engine?
Speaking of sbc bore and stroke (nominal dimensions):
283 = 3.825" bore, 3" stroke
307 = 3.825" bore, 3.25" stroke
327 = 4.00" bore, 3.25" stroke
350 = 4.00" bore, 3.50" stroke
383 = 4.00" bore, 3.75" stroke
400 = 4.15" bore, 3.75" stroke
4" bore allows for larger valves (intake and exhaust)
Longer strokes permit higher low end torque.
Spending $$ on the 307 is NOT what I would do. I would either accept it 'as is' (but IMPECCABLY tuned), or add at LEAST 4" bore and 3.50" stroke with complimentary heads/cam/carb/etc. I'd choose the 383 because it's not much more $ than the 350.