Jon,
I'm Ken..... up in the Florida Panhandle. Terry (Blueovalz) is correct, on all counts, regarding Porsche transaxles. Most of the "lighter Porsche" transaxles are designed with a very short first gear--and with that ratio in mind, use of first gear in a Manta (with V-8 HP) is pretty useless. Effectively, you end up running in four gears from a 5-speed transaxle.
Most of the later G50 and G52 boxes are beefier, and are probably a better match.
I'm running the Porsche Turbo (930) box, which is arguably the strongest of the production boxes. It's a 4-speed good for about 750 ft-lbs of torque; mine has the added plus of a good Quaife LSD in it. The 930's are an older box, and becoming somewhat scarce. I'd recommend you run it inverted, for a lower COG; but if you go this route, recall that your ground clearance will able be reduced. A good 930, modified to run inverted, will cost around $10-13K at current prices. The Renegade Cars website (Las Vegas-based) used to have a good comparison of the various Porsche transaxles; you might want to see if it is still available online. In addition the California Motorsports website (Lake Havusu) also has good info, and they are great to chat with at their 800 phone number. In either case, if you want to run the Porsche transaxle, I'd recommend you check out Kennedy Engineered Products (KEP) for the engine-transaxle adapter and clutch parts--quality items.... You'll also have to modify the half-shafts and suspension to accept the transaxle. In my case (as well as Terry's case....) we are running custom coil-over suspensions; in my case, I've adapted Corvette parts for the hubs/carriers and brakes).
Where in So FL are you? Santa Rosa Beach, here. If you are in the neighborhood sometime, drop me an Email at
keninflorida@aol.com