Howdy Mavdog, I'm running 89 C4 front and rear in my 54. Using the Progressive Automotive front cross member kit and home built in on the rear. I think once you slid your IRS up under your frame you'll see why notching might be a good idea. As you know the 54 frame rails in the rear are almost flat where as the 47 to 53 rail have a bet more radius and the later models are also narrower than the 54. When I slid my IRS under my frame I had the clearance of a stock truck. I've included the link to my build hope its of some use to you, and please feel free to message me I'd be happy to help if I could. Also if i were to do another one like this I'd make a narrower rear clip and weld it in. hope this helps....
http://www.67-72chevytrucks.com/vboa...d.php?t=419646