To begin your journey ….. (assuming the front clip is off)
I'd get the stock FRAME side engine mounts from a 71-76 Caddy (not Eldo).
Then bolt on the engine mounts to the motor. Attach the frame mounts to the
engine mounts. Then dangle the whole Caddy assembly in the frame nice and slow
to see if it's close to fitting. I bet some fab work to the frame or the frame side mounts will get you in there pretty darn close.
With the timing cover and HEI up front on a Caddy … you'd be surprised how close you can get it to the firewall & leave room for the Radiator/fan, etc. This also puts the greater percentage of the engine weight behind the centerline of the front axle for optimal weight distribution.
I put a 455 Buick in my AD panel and it fits great…even with a column shifter.
I'm guessing you'll figure it out … great choice for a motor!
and, yes … discs are a must. The Caddy weighs about 650-700 fully dressed up.

mike