Adding engine and suspension details to the AMT 59 Caddy Ambulance?

[Oldmopars]

So, I have built the Ghost Busters version of the AMT 59 Ambulance, and the latest re-issue is now on my mind. I know it is possible to add an engine and other details, I think I saw a post here about it. 

What is the best way to go about it? I think someone else used the Revell 59 Caddy. I have one, but with the cost of these now, I hate to cut it up for the AMT. I think I have a 59/60 Revell Impala chassis, I used the body on another project. 

The engine is less of an issue, I can always print one. But firewall, inner fenders, etc. What have you all done? The interior tub is an issue, but I did a regular Ambulance a while back and just rebuilt the tub to remove all the GB stuff.