We used a 30 ton bottle jack and 2 steel pipes against the ceiling of the 8 storeys flatbuilding the workshop was in and it came out.
Eventually.
Be very careful - you are putting some stress on the various bits..
In regards to galvanizing a used chassis : we have done it many times - as in : 8 - 10 - and never problems in the last 25 years. Have it blasted, chemically cleaned in bassins and drill a lot of 12 mm holes in the chassis to let the air out when it is dipped.
All the best !