Take the gen and heave it on the scrap heap until you want it's copper....
Take the original lower mounting bracket... turn it over end for end and then elongate the holes as far as you dare so it will slide forward. Then mount the alt... then you will see what you need to do to mount the top bracket. You'll also need a longer drive belt... mabey an inch or two. I forget exactly.
If your engine has the newer style plug over the oil pump drive you can update the instruments and install that drive. Otherwise you need a special alternator from Tisco that has a tach drive mounted on the end that is driven from the rotor. There are also Delco 10DN externally regulated alt's out there that have tach drives... but then you need to buy an external reg so that adds to the cost.
Wiring the 10si 3 wire is pretty simple. I run a 4-6 ga wire from the BAT terminal on the alt directly to the + Lug on the starter. Terminal 2 on the alt is jumped back to the BAT lug on the alt. Terminal 1 is wired with a 14 ga wire up to the charge light in the instrument cluster. When you cut the wire to hook into the light.... make sure that it's wired such that the light receives power from the ignition and the alt is hooked to the other side of the bulb. And you're done.
The instrument update is fairly simple also. Just remove the oil filter, pry the welch plug out that sits below it... then extract the oil pump drive shaft/gear. Install the new shaft/gear with the square drive hole, then install the new angle drive above it... hook the new cable up and the new cluster. IF you get the right cluster from '76-'79 or therabouts.... you don't have to change any senders. If you go with a newer cluster then the temp sender must be changed out and that requires drilling and tapping the head to receive the newer style sender.
You're probably into 500 bucks to do all of this when it's all done. That's only the cost of 2 gens and 3 regs. LOL. Or more commonly 1 gen, 1 reg and 1 battery.