plenty of E bodies have had v-10 pit in them,,not sure if they moved the fire wall or not,,,
A V-10 motor is based off of a smal block 360 with 2 extra cylinders,,so do some measureing on your current smal block,,,measure from one cylinder to the next,, then add that measurement to your V8 in length,,,that wil tell you how long the V 10 IS,,,you may be able to move the motor forward a small amount to advoid moving the fire wall back,,you may have to have a custom oil pan made,,or go with some ones after market front K frame kit,,that will give you all the under clearance you need to instal it if it clears the fire wall,,,but make sure you robb a bank first,,,