I had a 550 Spyder from Vintage with a 2.0L Subaru Turbo JDM. While the power was nice, the sound was wrong for the car. I think that the sound of 2.5 L NA might be better suited to a 904 and/or 550.
The bigger problem with the 550 was the suspension (scary at speed) and the brakes.
The car was too fast for its own good. That is the reason for modern suspension, brakes and engine on the 904.
You can see and hear it here YouTube - Vintage 550 Porsche Spyder - Subaru 2.0 Turbo II.
Strangely enough, I have a lot less in the Cayman engine and trans than it would cost to do a hopped up VW or subaru or 3.0L air cooled engine.
As for the delays, while I would like to have it done faster, I also realize that this is a one-off with a lot of trick stuff.
Sounds pretty good Mike. Well, at least fairly authentic.....just like a porsche air four banger engine, lots of clatter and valve train noise, etc.
I like how you're really babying the car and keep the revs down. Kidding.
Cool projects Mike! Keep the pics/vids coming!