Hi mate,
A P6 is not that big against a Bedford 28hp, Perkins did kits to fit a large number of vehicles in the 1950's. Biggest problem with a QL is making sure you have adequate clearance between the diff and the sump, exhaust downpipe, etc. There is a fairground operator in Sussex who has used a QL with a P6 engine for years, and is still using it.
A friend in Australia, fitted a later Perkins in his QL, the 6.354, which is about 5.4 litres. It caused all sorts of problems, due to its size and configuration, exhaust downpipe is above the diff, no room for the handbrake lever, so as it it has a compressor, brakes were converted to air over hydraulic and a cylinder was fitted to the rear axle for an air handbrake. Radiator had to be moved forward, I think. Then you also lose speed due to the lower rpm of the diesel, so larger tyres were fitted, also a 5 speed Turner gearbox.
A QL is a beauty to drive with the 28hp, just a bit slow on the hills, but once over the top, its away again.