Is there any reputable Mopar builders out there that could build a 440 stroker short block assembly and guarantee the rear main seal not to leak? I plan on using my cam and everything else from my other engine.

Nope! The rear main seal cannot be guaranteed on a factory block with an aftermarket crankshaft. Aftermarket cranks are not all machined correctly so that is one variable. The other variable is the machine work on the factory block. Sometimes the rear main area isn't machined correctly and you can't get a modern seal to stop leaking. The third variable is the seal itself. The factory used rope seals which are no longer available. Now everything is a lip seal and they may or may not work depending on how the seal area on the crankshaft is ground.

Even highly experienced shops have problems with the rear main seals on classic engines. They all leak a little, some more than others. Ford and Chevy were smart enough to redesign their V8 engines for one piece rear main seals but Mopar didn't do it. Mopar could've gone to a one piece seal when they did the mega block casting but they didn't and now it is probably too late.

if you use a good brand of adhesive silicone, not gasket maker silicone , to seal the rear main seal holder against the block and let it cure overnight, 24 hours before installing the oil pan it shouldn't leak.
I use a thin putty knife to smash the sealer into the seal holder joints to the block and the same on the bottom oof the seal holder to the rear main bearing lip on the rear main bearing boss on the blocks

I am retired but still build an engine now and then. I have built 100's of big block Mopar's my first was in 1973 a 383 for a roadrunner. In looking back maybe 5% had issues with the rear main leaking, most of those were modern builds not back in the day. Some of the issues with guaranteeing a main leak is you have no control over what has been done to the block, If an engine has been line bored many times that will cause a leak as the seal cap is still in the same place. How much did they cut? Never had a machine shop ask for the seal holder so it could be properly machined. Second as mentioned the machine work on modern replacement cranks is not the same as the OEM. And forget using a rope seal, not because they don't work but they don't make decent rope seals anymore, at least the ones in the engine kits I get are all junk. The rope is not cotton but something else, it will not conform correctly. Third having worked on cars my whole life when someone demands an absolute guarantee they are the type of customer that will cause you issues and most shops just roll their eyes and don't want to be bothered with it, plenty of customers that are easier to deal with. Don't mean to insult anyone that is just life today. I have a dyno and I run my engines for my customers if it does not leak on the dyno that is my guarantee, what happens after that to a performance engine is the customers responsibility. And trust me I have seen some customers do some stupid crap in my life. So that is the view from the other side of the coin.

I must be the luckiest man in the world.

Still no takers……

….this reminds me of work when people would bring 4.0 jeeps in complaining about oil leaks.

Conments made so far point to a missing piece of machine work at a lot of shops. The rear main seal to retainer on any motor in for assembly should be checked for proper diameter, just like rhe main caps. Nobody has ever done that when doing block prep for me, and now i realize i need to use a dial bore gauge and check that myself. I have had, over the years, a couple of leakers that made me tear my hair out in frustration .