Back in the day (my day at least), these were the Champion spark plugs to run in a big block. Is it still the same today, and where's the best place to buy them?

Get new old- old stock on ebay. In this style box. Champions kinda went bad somewhere in the 80s iirc.

Probably not going to find them now due to smog rules, resistor plugs only now
I thnk that champion quit making the J11Y and RJ11Y a long time ago, RJ10Y, RJ10YC or RJ12Y, RJ12YC available now

I would suggest the appropriate NGK. I've had to many problems with Champions.

You might be ok with NOS ones, but if I buy something that has champions those get chucked right away for NGKs. Have solved a lot of running issues getting those out of the engines.

New old stock is your only source they stopped making that plug decades ago.

i like j11y's. stocked up on them years ago. j11y and j13y were designed specifically for the mopar combustion chamber. j11y tip gets a little further in the chamber. i've used ngk's and autolites too but their no better than the j11y. only down side to a j11y i can think of is they're fairly cold.

So what is the big block choice of plugs nowadays?

probably the autolite 85 or the ngk thing. rj12y's aren't very good. not sure AC numbers would work but just make sure you use a projected tip. i really don't buy anything buy brand name exclusively anymore. and sure don't follow the internet/magazine herd. i just look for what fits my needs and works and forget about names on the part.

I run NGK BKR5ES-11 with aluminum heads for what that’s worth.

Autolite 3911 race plug or the resistor street plug # 3923,3924 or maybe the colder 3923 or 3922 with aluminum heads now

if the OP is asking about j11y's then i'd have assume he's referring to 14mm/3/8" reach/projected tip plug for iron heads.

This has been posted here a few times. I use NGK -6's for street driving and have moved to -8's when I'm spraying.

Becareful with the Autolite 85, new isn't the same as old ... that was my go to plug for a while ... when they moved production to Mexico years ago the quality went to poo poo , electrodes offset in the porcelain.

Mark I'd look at the NGK listed in the photo Nick posed , XR4 , or go on fleabay and find a set of the Mopar branded NOS plugs 34P , P34P , they are the J-11Y equivalent.

I ran autolite 85s for years…then Mexico happened.

i have a set of N-12yc's i don't think i'm going to use.
pm if interested. they are on the shelf for now.

The N series Champion plugs are 3/4 reach, the J series are 5/8 reach, not interchangeable

right you are Cab. my bad.
sorry.

J series threads are 9.5mm which is closer to 3/8 inch.

Ich nix verstain metric