The first Eldorados from 1967 and up to the 1985 I believe, while front wheel drive, had the engine mounted longitudinally.
Due to space saving, the engine was mounted sideways after that (following the trend of other car manuf/smaller hood design).
The last year of the Allanté, it had a Northstar... mounted sideways, coupled with the 4T80-E. Due to lagging sales, the Allanté was discontinued that year. Soon other Cadillacs had Northstars... but these Devilles, Sevilles, and Eldorados had enough room in the hood to mount them longitudinally....
Was it a money-saving scheme? (the fact that the setup would've had to be re-engineered?)
My speculation is that the N* was designed to go in the Allanté which did not have the room for a traditional longitudinal mount... and it took too long to come up with the 4T80-E and N* duo to just scrap it because the Allanté didn't make it.
There are some packaging and efficiency advantages with transverse mounting of the engine in a FWD application. Since the power flow does not have to "turn any corners" there is no high friction right angle gear drive. The hypoid style gears used for rght angle drive (typically seen in the rear axle but the longitudinal engine FWD Eldorados had a hypiod gearset also in the final drive) are a high friction device and cose fuel economy and generate heat.
In the transvers arrangement the crankshaft is parallel to the shafts in the transmission (more properly called a trans-axle in this situation) so chains can be used to transfer power from the crank to the trans. The drive axles run off the same centerline as the main shaft in the transmission so that there are no friction adding gearsets or right angle drives. All this makes a more efficient powertrain package due to the friction reduction.
Since all the powertrain in the transverse FWD cars is mounted on the engine cradle it makes it much less complex to build the car as there are no long driveshafts to the rear of the car and no rear axle delivering power to the rear wheels. The car also does not need the heavy structure in the rear to contain the power delivery from the rear wheels as the cradle/subframe that holds the engine and transaxle also contains all torque reaction of the powertrain and tranfers the motive force from the wheels to the car to make it move.
Transverse powertrain packages can be designed and manufactured very compactly with the trans tucked beside the engine. It also allows for good powertrain structure by attaching the transaxle directly to the engine block where it mounts besides it.
The Northstar engine was designed for transverse mounting from the very beginning. It was highly optimized to package in as small a fashion as possible for the transverse mounting. That is why the water crossover casting is on the rear of the engine mouting the coolant passages into and out of the heads and block and why the water pump is at the rear of the engine. It is much more efficient to package it at the rear of the engine in a transverse setup where there is room over and around the transaxle bellhousing that is unused otherwise. With a clean sheet of paper engine like the Northstar the cooling system flow and ports could be designed like this from the beginning since the engine was not a longitudinal engine converted to transverse.
The most telling factor that shows the transverse optimized design of the Northstar is the fact that the right bank of cylinders is staggered ahead of the left bank. Most longitundinal V8 engines had the left bank of cylinders staggered forward. Since the right bank was up against the firewall anyway and there was no room for accessories on the right side the right bank was staggered forward on the Northstar so as to leave as much room on the left (front) side of the engine where the accessories would be. This is why the Northstar has number 1 cylinder on the right side and not the left.
Understand that there is no "perfect" powertrain mounting or architecture. There is a case for longitundinal mounting as well as transverse just like there is a case for front wheel drive and rear wheel drive. It all depends on the application and mission for the vehicle. For a passenger car the transverse FWD architecture is very effective as it takes the least amount of room and it is very effiient without the right angle drive in the system.