Switch 4.5 to 5.0 in 88 Caddy?

I have an '88 Caddy with a 4.5 litre engine and I have been told that my head gasket is blown. The mechanic is saying that it would be cheaper to switch engines than to fix the head gasket. I have the opportunity to buy an '88 Fleetwood Brougham with a 5.0 engine that I am told is fine. Can the 5.0 take the place of my 4.5 with little or no hassel? Thanks for any immediate replies as I have to buy the '88 Fleetwood today or tomorrow or the person is selling it to the wreckers.

No it would require a lot of work if it even can be done.