I have a two piece rear seal on my 427 and I stagger mine as well and I use just a tiny dab of silicone on the back side. No leaks there but I have had trouble in the past getting my intake completely sealed up.
$1500 is extremely high and you need to look to another shop for all your work. Everyone needs to make a profit to stay in business but that is a lot of money for your issue. I recently pulled a friends engine in about two hours (just the engine - we left the transmission in the car) in about two hours. After that you would only need to mount your engine to an engine stand and rotate the whole thing over, remove the 20 or so pan bolts, then remove your 5 main caps, remove and replace the rear seal, re-install the main caps and torque them down then re-install your
oil pan, and stick it back in the car. All that took maybe an hour after getting the engine out of the car and the seal cost about $10.00. New
oil pan gasket another $12.00.
In short the whole job shouldn't take more than 6 or 7 hours and at the very most should not cost more than $500. IF you do it from under neat then $150 should be about right.
Clois