I agree with David here. A good strong bottom end is the best place to put your $ now. A mild cam, a 252 / 260, will improve torque some, I don't know about mileage. I think not. The stock cam is 252, but low lift. The "performance" version by Comp, Isky and others has higher lift, which will contribute to performance. By the way, Clifford is a good source for quality product and customer service is improving. Pay by credit card and if you don't get your parts you credit card issuer will charge them back for you - no choice on their part. I pay for everything "mail ordered" this way.
Be careful about "decking" and "milling" without a plan. Know what compression ratio you are going for. You will not be happy with a compression ratio over 9:1 in a daily driver. As a matter of fact, if you want to use inexpensive fuel (87 oct regular), a little under 9 would be good advise. If you are willing to buy premium fuel and upgrade your ingition (a separate subject), a little over 9 would be a fine (note: I'm talking a range of 8.7 to 9.2 here).
Carburetion. Unless you are able to sort one out after its installed a carb off of a 300ci motor is not recommended. You can over carb the 200 quickly, and flow alone is not magic. There are no "bolt on" upgrades. After the engine is done, you can play with carbs, headers, etc. til your hearts content.
Head. Do get a "three angle" valve job. Do install adjustable rocker arms. Do get hardened seats.
Get a copy of the Ford Six Performance Handbook before you start. Look it up on the web.
Have fun with this. It's the best part.