Expect engine will have 620 - 700 HP. Milodon's new main cap offering shows long wait time. What else to go with?

Steel or aluminum? I had BCR caps with a girdle, affordable and nice as far as I could tell.

stock, caps ain't the problem.

Whatever works best. Was trying to forgo a girdle and any problems with clearance and such. Do you have a link to what you recommend?

So just go with the original caps?

I have had 2 cast iron block motors. The 474 was a 400 block with the factory caps and an offset ground factory crank, It had 600 passes and I wore out the cylinders. I had a 440 cast block with aluminum caps that had a 4.375 stroker crank. It had a ton of passes. I put 300 on it and traded it to a guy who ran it to. My opinion since I have ran both is it doesn't matter as long as you have a straight line bore and good crank and the right bearing clearance. Birdtracker

But it’s not as cool!

I've built a bunch of BB Mopar stroker motors (pump gas street and strip and drag race only on race gas) with stock 400 and 440 block as well as few with after market race blocks, skip the girdles and use a set of after market main caps, ductile iron, aluminum and maybe even steel caps now that you can't buy the ductile iron main cap kit from Mopar anymore
Make sure and use a good set of studs and have the block aligned hone to get the new caps to fit properly
I've made over 820+ HP on race gas with a 440 block prepped(526C.I. with CNC ported 440-1 heads 440-3 single four cast aluminum intake manifold and a older Holley list #9375 1050 CFM Dominator carb on 112 octane race gas) that way years and years ago, it is still running with no issues according to the owner I sold it to
If you look at the stock BB Mopars and Gen 2 hemi blocks they have a lot of stock cast in main bearing support hanging down to the oil pan rail, no girdle or 4 bolt main caps needed
EDITED, use a good 400 block after having it sonic tested first for all out high HP(700+) and higher RPM, above 6000 RPM due to the stock 440 blocks and the other stock RB blocks cracking the main webbing above the main bearings up to the bottom of the cylinder walls, don't ask me how I know about this
Buy a new race block is the best choice to start with if you can swing it

I had Herb McCandless tell me that a high HP stock block either needed aluminum rods or aluminum main caps to survive. On a stock block original main caps at the HP levels you are asking about, my experience is it will break number 2 main cap, not if but when. Never had an aluminum main cap fail. When I used a girdle I went with steel main caps, as with a girdle you are trying to make everything ridged as possible. I have used 440 source steel main caps with no problems. With steel main caps and a girdle I hod one engine barrel the cylinders, and it had block filler in it, the cylinders pushed so hard it cracked the block between the freeze plugs, about 500 passes on it. The next one cracked the webbing between the number 2 main and the cam journal, never blew up just found it on a freshen. Bought an after market block, best investment I ever made, take it apart after 300 runs looks like brand new. I know they are expensive but no way I would ever try and make 700HP with a stock block again, not worth the risk of blowing an engine and wrecking the car. Back when I was doing it nobody made an after market block.

http://store.440source.com/Main-Caps/products/25/

https://www.ebay.com/itm/390768907780

I bought a used 440-540 short block that has billet caps on 2,3,&4
I’m pretty sure they’re CAT. It had fully ported edelbrocks and a fogger
hidden under the intake. It ate some lobes on its purple shaft and killed
the rings and skirts , but the block was fine - magged good

Those new Milodon cross bolt caps look pretty beefy.

https://hotrodenginetech.com/milodon-440-chrysler-main-cap-upgrade/

There is one very IMPORTANT item nobody has mentioned. Keep the motor from DETONATION! Nothing will kill it faster!!

And to add to that bit of esential info, here is what i would build. 400 block, aluminum main caps, studs, aluminum BME396 forging rods, and pistons with . 043 rings. Fill the block part way with Hardblock. Geting the bob weight very light will help a great deal.
An expensive investment, but one that will take a lot of stress out of the short block is a center counter weighted crank. That evens out the loads on the main bearings, greatly reducing stress on the block.
I would consider going hbeam rods to save $$ toward the expensive CCW crank. I have been running the same CCW Crower crank since 2005.

Yes, The block cracks, when they do fail. Put some arp studs in it if you feel compelled. New bolts if not. Cycle them a couple times(which typically occurs during clearance checking). I think some blocks are prone to crack the day they are born, if they are lucky enough to to get pushed hard. Others live long boring lives.

I also would consider the block as a replaceable part of combo like that. So I wouldn't dump a bunch of money in it. Give a good check over every 2-3 hundred runs.

What block are you working with ?

Studs and aluminum caps, keep it out of detonation and let her rip.