The vehicles you refer to are still in service, and the cost of aluminium armour is considerably higher than rolled steel armour per kg. The J-60 engines are now replaced by diesel power in keeping with the fuel used by the remainder of the British Army.
If you wanted reliable power from an easily available powerpack in 1970, during the design process, which other British engine would you have used? The B-60 was too old and too heavy, the B-80 just too big, and the R-R V8 was too wide to fit in the space. The J-60 was an excellent and reliable unit, and who else had a twin overhead camshaft tank?
The fact that these vehicles have lasted 35 years indicates that aluminium is not such a poor material from which to build tanks if your design capability, welding knowledge and manufacturing engineering are sufficiently advanced, ask the soldiers now serving in the middle east in Warriors.
john from Staffs
retired GKN Defence and Alvis employee