Nice job so far Hugh!
About that bumper, to do it correctly I think chrome plating or alclad is going to be your only option.
I'd section the bumper by cutting with a razor saw right thru the middle of the rubstrip. Use masking tape to protect and act as a guide. Then block-sand down to the proper dimensions, getting rid of the strip. Round off your cut-lines to reproduce the stamped-in look of the 1:1, and polish the bare plastic with at least 1200 grit paper. Leaving the plating on during this process will give you a good guide to be sure not to take too much off & keep everthing straight. Then re-attach the upper & lower sections with strip styrene from behind, creating your recess. You may have to thin the bumper from behind also for the correct scale thickness prior to gluing. If you take your time & prep the two sections properly, very little finish work will be required prior to replating or alclad. Strip the chrome, prime, and check your work. something like Mr. Surfacer brushed into the recess will fill any voids left from the surgery, followed by more sanding
A bit of work, but worth it for the accuracy! A spare bumper couldn't hurt either!
Good luck!