New Fuse Box, but no wiring harness

[bobwires]

I've got a '78 I'm working on, and the customer brought me a 'Painless' harness #10203, 18 circuit GM. The fuse box isn't the same shape as the stock one....








my question is this: Has anybody just replaced the fusebox alone? I'm confident I can do the job, since I solder wires for a living, but I'm looking at the pros and cons of chopping the wires and having a bunch of heat-shrinked wires there vs. ripping it all out and doing the whole job.

Also - I could keep the fuse box plastic, and replace all the fuseholding tabs and wire connections in the back. that is the problem on this truck - the wire is fine, no green, no stiffness, but the fusebox is rusting out.... sucks for me.

any thoughts?