The first and most obvious reason is efficiency. You will get more power using less fuel with superior drivability with a port injected engine. In a early Mustang the potential exists to get up to 28 M.P.G. highway. Although the initial investment is somewhat pricey, the payback is in better gas mileage from that time forward. Depending on how many miles you drive your vintage classic, the difference in mileage is worth penciling out for yourself. As far as cold weather driveability, there just isn't any comparison. Now lets talk performance; the stock five liter injected Mustangs made 225 H.P. and 300 FT. LBS. of Torque! Think about that for a minute. Three hundred foot pounds out of a 302 with a measly 3 inch stroke.
What all this means is you have more torque than a 1969 four barrel 351 Windsor; and this spells a light car that is fun to drive! Pull away in third gear with a carburetor? I can only try and relate this through mere words, if you have not driven one of the late Mustangs yet, words alone probably won't be sufficient. Very likely you have been beaten by one at a impromptu boulevard stoplight grand prix.
Enough of this, hopefully you get the idea by now. What I would like to attempt to explain is why the swap cost is so much higher than simply bolting a high rise manifold and a 4 barrel carburetor. Most people seem to understand and relate to the necessary expense of the electronics and sensors and the like. What they do not consider and tend to overlook is the fuel delivery system. A typical "stock" fuel pump runs anywhere from $235 to $295 and the high pressure fuel line is nearly the same cost. Now you have to figure a way to adapt these to your vintage car and find out it's just not as simple as it seems. You have your filters, suction, and high pressure output and how to make all this work on a vehicle that was built over thirty years ago. It has to be safe; a typical port injected engine has a fuelpump capable of outputting up to 90 P.S.I., unregulated. You wouldn't want to use a garden hose for this, after all your family will be in this car.
There are a number of things that you will also need to consider and address as well. What about a fuel safty cutoff or "crash" switch. Don't forget the exhaust system, you will need oxygen sensors and fittings to accommodate them. The late models use the superior "serpentine" accessory drive belt system; this means the water pump is reverse rotation, and that the inlets/outlets are reversed and the hose sizes differ. You will also have to come up with a shock tower brace, since the stock one will not clear the EFI upper manifold.
Then of course is the wiring. Unless you are an engineer or electrician, no doubt you will be money and frustration ahead if you get professional help here. I can tell you that our company is not getting rich doing the wiring harness work. After spending 3 1/2 to 4 hours on a harness, plus the parts involved, I'm working for wages! Seriously, we offer this as a service to our customers, and provide telephone support, should you need it. I feel our prices are quite reasonable considering the amount of R&D that goes into each product and that they are designed for bolt on installation for any average enthusiast. I started this business simply doing the wiring part, with my background as an industrial electrician, this part just came naturally. However, more and more I became frustrated by the poorly designed products that companies were getting big money for. We began doing more and more R&D into products that fit and work, and not doing it as a "sideline", making up parts whenever they needed to "fill in" for their regular work. No sir, we did the actual swaps ourselves and it was immediately clear that there was a market for quality pieces, designed by people that understand all of this and have a commitment to the swap business, our only business. The rest is history, we have been in the swap business now for over ten years and have customers in almost every continent. Thanks for letting me share some thoughts regarding fuel injected engine swaps.