I have "N5U" listed as a GMP&A replacement block for a 308 blue motor, not a 304.
I thought the "N5" prefix spare parts blocks were replaced by "S" prefixes for VN through to VT?
"SVU" & "CVU" prefixes came from VQ-onward V8 Statesmen and Caprices respectively.
I believe you may be talking about the "SU" prefix which was a GMP&A replacement 304 block around the VR / VS era?
Adding to your info
Yep N5??? number blocks were 308 short motor replacements up to WB including VB - VH
However other N5 ?? numbers were used in the VK/VL as 304 short motor replacement blocks in the changeover from 308.
SV was for VN – VR 304 replacement blocks. SU was for VR – VS 304 blocks were a roller block with no 304 cast on side of block. Statesmen were SVU numbers except for HSV models. All other VN – VS standard 304’s were VU
The OP might have a 308 block ?????, the casting date sundial pointer for 308 block was in the 9oclock position and the 304 is in the 1oclock position.