To my knowledge, the only re-engine STC that provided a legal gross weight increase (normal category operation only) was Aircraft Conversion Technologies. This old post from the Taildragger Club website says ACT has no website (I can't find one) and gives an individual as contact information: ACT Information [taildraggerclub.org]

Another site has this to say: "Aircraft Conversion Technologies of Lincoln, California owns the Texas Taildragger STC."

The FAA site gives this info: "Aircraft Conversion Technologies, Inc, P.O. Box 119 6245 Aerodrome Way Georgetown CA 95634 United States"

In another thread, Jeff Davis has this to say:

Quote
Mine was a Bush/Williams/Avcon and DID NOT have the GW increase (a piece of paper that ACT does have and will sell for about $1200...no other changes).


I've no idea where he got that information. Unless your 150/150 has the ACT conversion, I don't see how their paperwork could legally apply to your aircraft, or any other companies STC. If you already have the ACT STC, I would think you already have the gross weight increase.

Carl