Engine & Transmission Compatibility

A '90 is not a Magnum, it's a roller LA.
It's neutral balance, so you may be able to make the flywheel work- I'm not 100% sure, but I recall the 273 having a smaller crank register (OD). You may be able to have your flywheel bored to the later diameter or find a later 273 flywheel to bolt up.
All the 273 front end parts will bolt right up, including timing cover and accessories, you'll just have to re-mark your balancer.
'65 and earlier intake manifolds won't fit '66 and later LAs.