Good lord, I thought my Speedster body was in a bad way. Do you have the roof?
Without it it's a lot of work to either make one or convert the rear deck to the Speedster/Convertible shape as they're quite different. Even the engine grille is a different shape - flat on the coupe and curved on the Speedster and Convertible.
As for the chassis or re-body, the Triumph Spitfire wheelbase is only 8mm longer than the 356 - 2100mm vs 2108mm while the rear track width is the same (with 1500 half-shafts) at 1270mm. The front track width is also very close - 1306mm (Porsche) against 1245 (Spitfire) - so nothing that can't be sorted with spacers or wheel adaptors.
Obviously, the Spitfire chassis option is completely off the wall, so you might want to consider a Pie-Valley chassis with MGF/TF mechanicals -
http://www.pievalleybuggies.co.uk/beetle-kitcars.html
There are lots of positives with that chassis, not least of which is the choice of engines from a 1.4 K series right up to the 2.5 KV6 and really good brakes, but there are two big negatives for me:
IVA
The MGF track is much wider - by 94mm at the front and 140mm at the back - so flared arches might well be necessary which aren't to my taste.
I could be wrong on that one, though, so do speak to Pie Valley if you're interested.
Anyway, nice to see another crazy Outlaw build commencing.