1.6lTD long runner intake manifold idea, take 2:
Instead of putting the long runners in the factory location behind the motor close to the firewall, another option is to make the runners go over the top of the cylinder head, like this:
A test fit revealed I was able to close the hood down all the way and there was still a little bit of room. Although the throttle mouting flange clears the hood, I might want to cut it up to get a little clearance with the valve cover, and perhaps position the plenum down a little bit more.
To make service under the valve cover easy, the intake manifold should be a 2-piece design, with all the bolts for the upper piece being accessible without needing to remove the turbocharger. The upper piece could be an A1 gasser manifold that is stock except for a few unneded protrusions cut off, while the lower piece could be a special fabricated piece made from the bottom of an 1.6lTD intake manifold that allows the A1 gasser manifold to bolt up in the proper position. It would start with this piece:
Compared to the other long-runner intake manifold design, this would leave the exhaust manifold and exhaust system, turbo location, air intake and compressor outlet tubing, oil feed and oil drain, etc, all unchanged. The boost tube run from the front mounted intercooler would be shorter with less distance to travel to get to the plenum. The intake manifold also wouldn't be positioned over the hot turbocharger but in a location that's a little bit cooler, requiring less heat shielding.
As a con to this design, it uses the 1.6lTD intake manifold's tighter radius runner bend, which might restrict airflow somewhat versus a design putting the plenum behind the motor. I wonder if it that tight bend would make it much more restrictive.
Also due to more limited clearance, I wouldn't be able to easily substitute or test various types of long-runner manifolds (In particular, the A2 manifold, which has longer runners than the A1 manifold, would stick up too far and interfere with the hood.)
I have had trouble oxy-welding thin tubing (.065" tubing) but have had good results with thick (1/4"). But I need to see how well I can do cast aluminum. I'll need to do a little experimenting first to build my confidence. I suppose at worst case, I could always prep the parts for welding and take them to someone who can TIG things together.