I'll expound a tad more...
A 914 tub is a 914 tub. The better, the better (all the circle talk ties into the "I'd do what I did" comment)
I agree with George on the -6 tub thing for the most part. There are two cases where I think it could net you more to use that tub in the end:
1) Build it out as a stock 914-6. Something you're not looking to do at this point.
2) Build it as a "Dead Nuts/Spot On" GT clone. That would mean no 3.6 and no full cage etc. I've seen well done GT clones (Peter's car) go for big money and I think the -6 numbers had a lot to do with that (not to mention the flawless execution of the build).
For what you would like to do (going off your list), I would contemplate either lowering the price of your tub so someone else can take on the task of bringing it back to a rare 914-6 (one of the most limited "production" cars ever made by Porsche) or, keep the tub and start saving money and parts to take on another 914-6 project some day (this may not be your last if you're like many here). A few hundred bucks a month and in 5 short years you've aquired $12,000 worth of original 914-6 parts. Most are easy to find.
If you lower the price of the tub so someone else can restore it, you could put that money into a lot of metal work on a very straight clean CA -4 tub. $3-4k would go a long way toward getting a tub paint worthy (flares, stiffening, cage, misc. rust repair, etc.) at which point all you would have to do is have it painted and start bolting on parts. There is really no advantage in doing this to a -6 tub vs. a -4 tub and in fact, as mentioned in this paragraph, the -6 tub, with it's serial numbers, could go a long way in helping your project along.
You might want to take that approach a step further with "Bird in Hand" thinking. Buy a driver so you're not 914less... sell the 914-6 tub and use that money to get your current 3.6 car up to the level of prep you want. As you go through it, change the things you want changed, fix what needs fixing and clean and restore the existing parts you have that already makeup a good portion of what you're looking to do. Put it back together and enjoy. Then sell the daily driver you bought (for a little more than you bought it for). You'll have saved an old friend who's served you well and... you already have it running so not much need for more parts etc.
I think you've had plenty of "it's your car" advice so hopefully this will provide a different side of the coin to look at. While I'm not a die hard "purist" I do recognize there is value in a tub in which the serial number begins with 914. I'd leverage that value as much as possible.