that 1407 can flow 750+cfm when tuned.. that is more than enough; even at your altitude.. (i still love a good Rochester, but back when edelbrock were putting together their dyno packaged performer 'power packages' line of carb/manifold/cam/lifter/heads, etc.., they spent millions in dollars and hours of dyno and field tests to insure when they posted dyno results, it was VERY close to what a DIYer could roll out the garage with hand tools)
if your 425 was like my 403Rocket, you should see 5-7psi..
if it was a performance (hi-volume or hi-pressure carter/holley) pump, you may see 8-12? tops; unless you add an electric pusher pump, then you would push into teens/twenties..
as far as clearance; cheap trick (not the band), flip the air filter lid over..
still creates dust seal on top of element, but can give you an extra 1+" in clearance, as the dome of lid rides inside filter, instead of above..
also, some 4.5/4.9 cadillacs used 3" risers in between the carb and element, so the OEM filters cases were inverted parabolic; and dropped over the carb.. (if you track one down in salvage, or buy an aftermarket; just insure it clears all linkages)
and if you don't want to drain the bad gas out of tank (i wouldn't), don't forget to add some premium to tank, or B12, or BG44K; otherwise you'll be going through alot of nylon fuel elements..
hope this helps..