The '70-71 parts catalog (Jan 1972 edition) lists shroud part number 2998326 for '70 and '71 B and J (Barracuda and Challenger) 340 with 26" core, with or without A/C.
The same part number is listed for '70 and '71 B, J, R(Belvedere/Satellite), and W(Coronet/Charger) 318, 383, 426, and 440 with 26" core and A/C (yes, 426 was listed here)
There's no listing for a non-A/C 26" variant for those models though so maybe there's a typo, or did any of these cars come sans shroud?
So there was overlap on the shrouds for 26" core radiators between LA and B/RB engines, at least with air in 1970 and 1971.