Has anyone replaced a 4.1 with a 4.5 in an 87 Allante? Is this a feasable swap. Will I need other parts from the doner car. (computer, harness etc.)

 

it has been done. it hasnt been cheap, easy, or trouble free.
for the work involved, you be way better off to replace the whole car.

if your '87 is in really good shape, part it out .. the revenue from the parts will easily buy a 4500 model, '89 to '92, in pretty good shape.

these cars have little residual value or demand.
however, the parts are in high demand.

CAUTION: be very careful who you take advice from. the Allante is a unique animal and the engines & controls are just as unique. one can go from an Allante to the other cadillacs, but often, not the other way around.
.

 

As Stealth stated it can be done, but it is not a simple swap. If you have a 4.5l Allante' as a parts car it will make it easier. The Allante' engine is not the 4.5 TBI engine with a different intake, So you would need an Allante' engine, and all 4.1-4.5l engine swaps I know of have had computer calibration issues, custom chip could be made. If you don't have a donor car it may not be worth trying.