After years and many gasket sets different pans ect. I have figured out the long time issue. The Neutral Safety Switch is the culprit! Two cars fixed in the last month and now both completely dry. Around $10 for a new unit.
I would look a modern Challenger setup and also do a bit rewiring to make existing headlight high/low beam and halo intakes and use both high beam holes.
You may want to check were the column passes through the firewall. Make sure the column is not hung up on firewall mount. The nylon pin is pretty difficult to break. I recall having a similar issue. I don't believe there is any difference in column length between small block and big block...
R22 would be on the fender tag for a 22" radiator. The 26" would not be indicated on the tag if equipped.
My 70 R/T 440 Magnum came with the 22" and it pretty much sucks for cooling. Modified to a 4 row core helped. It does have the R22 code.
One thing you should consider is the size of the pinion shaft for a 741 case is only 1-3/8". A 742 case is 1-3/4" and a 489 case has a 1-7/8". Just things to consider depending on use.
I would like to get $1,000 for it. It is in very good cosmetic condition. Does not function, like most of them. I have had it since 1978 when I purchased my 70 Challenger. I have another out getting refurbished and adding Bluetooth. I'm waiting to get it back before I consider letting loose of...