The 261 full pressure oiling will indeed fit the motor mounts etc. Your water pump will need to be adapted with the short shaft to clear the radiator, however this will make the original fan interfere with the lower supports. You can get an adapter to use an different water pump which requires an extra hole be drilled in the block or you can put a pusher fan in front of the radiator and eliminate the fan.
If you are going to add dual fenton exhaust, you will have to bring the oil filter lines out past the hot manifold with metal lines and adapt them to a remote filter. If you have the filter from the 261 with the large cannister for the PF 141 filter the housing may be cast aluminum and able to be polished. It will conveniently fit above the steering column on the firewall.
One of the past posting also suggest that you change over the clutch to the larger size (10.5 inch ?) to accomodate the extra torque of the 261.
