engine length

I believe that if you measure the total length of the V-10, you'll find it significantly longer than your existing RB engine. Fire wall/cowl modifications and/or relocated radiator or condensors for your Imperial would not be good or easy or inexpensive to do, no matter what. A cleaner alternative might be one of the stroker engines that are in the Mopar Performance catalog (with somewhat downsized cam timing), or just upgrading the engine you have with a more modern cam (not too wild, of course or the power brakes will not work due to lessened idle vacuum), one of the 440/375 intake manifolds, and some better exhaust manifolds and a 2.5" under car exhaust system with, of course, the 2.5" inlet/outlet '72 Imperial mufflers. Adding a Chrysler electronic ignition conversion kit would need to be part of the package too, for several reasons. Other than the later style valve covers, much of that could be made to look just like what you have now for the ultimate sleeper look. A good performance rebuild with at least OEM level parts would make sure the TorqueFlite was up to the added power too.

That covers the engine/trans. Others have expertise in the chassis and brake areas, but they will need to be refurbished/upgraded too to maintain the total basic package approach that Chrysler did so well with their vehicles. There's lots more to enjoyable vehicular performance than just accelerating!