As a guy who burnt 83% of his body and spend 16 weeks in a burn unit, along with about 40 yrs of serious related health issues since, a boiler accident. Remember the engineering of a boiler is extremely complex. I currently have 45+ yrs in the industrial boiler business.
While the propane tank has a high pressure rating, but at what temperature?? Even low pressure steam boilers (less than 15 psi) have shell thickness of about 1/2”, way more than a propane tank. Be very careful of unapproved homemade designs, building and property can be replaced, but human flesh is very unforgiving and the after effects can be extremely life altering.