440 motor home engine-spark plug?

I have a mid 70's 440 motor home engine in my 69 Charger. It has the stock heads that use the spark plugs with the beveled seat. Been trying to see what my options are for spark plugs, can't seem to find any info.

I think those are std Chevy plugs too.

Tapered seat spark plugs are your only choice, correct?
Are you wanting to know something else like spark plug heat range?
if so I would start with the spark plugs that the motor came with and adjust the jetting and timing to make the motor perform the best with those plugs as long as the motor is still stock

Cab. I don't know what plugs came with the engine, non running when I got it and they disappeared during rebuild. Been chasing an ongoing issue with the engine and a buddy suggested that I'm running the wrong plugs. Last night I finally found some info on a motorhome web board where they stated they are using Champion BL9Y or RV12YC. I am currently running NGK UR4-V power. So next step is to see if these plugs show to be interchangeable.
I was just wonder if anyone on here had the same motor and what the were running to give me a starting point for compatible plugs.
As far as my engine it's bored .30 over with stock pistons, only upgrades are a mild D/C cam, Weiand intake, and headers.

Stock 440 motor home pistons or passenger car pistons?
If passenger pistons which year and do you know if they are flat tops, flat tops with valve reliefs or what motor they were made to fit in?
the reason I'm asking these questions is to try and see if you can post or know closely what the compression ratio the motor has now?
That NJK #4 spark plug is a real hot spark plug, don't run it
If that type plug is a tapered seat plug with a extended tip see if you can a #6 or#7 plug number like that one .
If it is retracted gap, center part flush with the main plug body, then see if you can get a tapered seat plug with a extended tip in #6 or #7 heat range and try them

I ran the NGK UR5 in mine, eng. was in a Jensen & they ran hot,the NGK XR5 13/16 plug with gasket used in B/RB eng.is the equivalent plug, NGK heat rang the smaller the number the hotter the plug.

Spoke with the engine builder today. He's pretty sure the compression ratio is 8:1. He said there were only two choices for the pistons since we were just going with a stock piston, he was going to try and look them up tomorrow since I found his itemized bill. I'm fairly certain they are flat tops.
The date on the block is 4-11-78.
The Weiand intake is model # 8009, dual plane, with an Eddy 750 on top.
Camshaft is Direct Connection. duration 272, overlap 48, design lift @valve .455",

71GTX. Yes, I was reading that the heat range values for plugs differ with each brands and some are reverse of others.

Use the UR5s your eng.build is relatively mild.

Thanks.
Also running a Firecore RTR ignition system. Not sure if that would make any difference on spark plug heat range.

Having a stronger Ign.system gives you the ability to open the plug gap a little say .038-.040.