JTR at http://www.jagsthatrun.com sells conversion parts, and has excellent manuals that explain the Volvo 240 series conversion process and pitfalls. I advise you to purchase and study the JTR 240 series conversion manual, no matter what engine you decide to install. http://www.converseengineering.com sells a popular kit for converting 240 volvo's using a Ford Mustang 5.0 HO engine. As far as size and weight, there is very little difference between the Ford 5.0 and Chevy small block engines. This is all explained in the JTR manual. Each engine has good points and they are both very reliable. Your choice should be whatever engine you are most familiar with and comfortable working with. It is an interesting project that will give you great satisfaction when completed.