What you need to appreciate is that the 68 to 70 models years are highly sought after by everyone and the chances of finding even a frame or engine for that kind of price would hard to quantify. Sure you might get lucky enough to find the one owner that has lived under a

for the last 30 years but don't count on it.
Building the bike up bit at a time will be the most expensive way of doing it and you may then have issues making it legal.
By far the cheapest thing to do is to buy a complete bike and go from there BUT like with all things unless you school yourself up and learn to identify the right bits you will get ripped off with something pretending to be something it isnt.
Sounds harsh but its the truth.
In short you will need more cash and more knowledge. Alternately look at a decent 500 twin (68 onwards) as these are often overlooked and can be made to run very well.