You are missing that its a 4 cyl engine, hence lighter and is better suited to distribute proper weight, If I were to shove a 6 cyl into a celica, I would get a mercedes M104 (24V) engine, put a Motec on it and join a GM TH350 to its behind. and fool around all day long.
If you take off the turbos of a 1JZ and see the exhaust ports - you will be amazed at the sheer stupidity of Toyota/Yamaha when they were designing it. - Its more of a case of how not to make a cylinder head. But anyway, do show the firewall cut - and as you are cutting the firewall, ask the guy to cut the tunnel and make room for the exhaust too.