They've all got a tube subframe, Bruce, but what you've got there is special. The 911 engine makes it so.
However, I'm not really sure what you're asking. These things are titled many different ways, but you're never going to get more information that you've already got. Everybody titles and registers these things in one of three ways:
The one most guys use is to just use the VIN stamped on the VW pan (if so equipped). I have no idea if you have a pan or not. You indicated it's built on a 911 "frame"-- but as Wolfgang pointed out, 911s are uni-body cars, and have no frame. Unless this car was built on a custom pan-replacement frame (and some were, Metal-craft, etc), there's a VW pan and VIN. It'd be located on the tunnel, under the carpet, just in front of the rear "seats".
Some cars were titled and registered as "special construction" cars with a state issued VIN- yours doesn't have that, and I'd avoid going in that direction. It subjects you to emissions testing for the year of manufacture (the year they titled it), which your state may or may not have-- but if you sell it down the road, it's a big deal.
Some states have "SEMA titles". These allow a guy to legally title and register a car as a "1957 Speedster Replica", or some derivation on that theme. California has SB100 titles that are similar, but limited in availability-- these titles are really coveted, as they are completely legal, and allow a replica to totally avoid emissions.
There are guys who have used title companies to create Porsche titles out of clean cloth. There are guys who have taken VIN plates off wrecked 356s or 911s, and affixed them to their cars, and titled them thus. There are guys who have just made up Porsche VINs and filed for lost titles. I'd avoid all of these, as fraud is generally punishable by large fines and jail time.
Your car has a title somewhere. That's probably what you'd take to the DOT, or driver's facility in your state. It might not be right, but getting it changed would open a huge can of worms.
Forewarned is Forearmed.