The menu code is fine. Leave the six character spells where they currently are and redo all of the enemy spells you want to be six characters. For the name display in battle, you're in luck since the game actually has two different loading routines, so it's pretty painless to do this hack.
Here's the relevant code:
$02/CB95 AD C8 34 LDA $34C8 [$7E:34C8] A:0040 X:0004 Y:6400 P:envMxdiZC
$02/CB98 D0 01 BNE $01 [$CB9B] A:0023 X:0004 Y:6400 P:envMxdizC
$02/CB9B C9 48 CMP #$48 A:0023 X:0004 Y:6400 P:envMxdizC
$02/CB9D 90 25 BCC $25 [$CBC4] A:0023 X:0004 Y:6400 P:eNvMxdizc
; load eight character spell name ---------------------------------------------
$02/CB9F 38 SEC A:0051 X:0004 Y:0012 P:envMxdizC
$02/CBA0 E9 48 SBC #$48 A:0051 X:0004 Y:0012 P:envMxdizC
$02/CBA2 85 26 STA $26 [$00:0026] A:0009 X:0004 Y:0012 P:envMxdizC
$02/CBA4 A9 08 LDA #$08 A:0009 X:0004 Y:0012 P:envMxdizC
$02/CBA6 85 28 STA $28 [$00:0028] A:0008 X:0004 Y:0012 P:envMxdizC
$02/CBA8 20 60 85 JSR $8560 [$02:8560] A:0008 X:0004 Y:0012 P:envMxdizC
$02/CBAB A6 2A LDX $2A [$00:002A] A:0009 X:0004 Y:0012 P:envMxdizc
$02/CBAD 7B TDC A:0009 X:0048 Y:0012 P:envMxdizc
$02/CBAE A8 TAY A:0000 X:0048 Y:0012 P:envMxdiZc
$02/CBAF BF B0 8A 0F LDA $0F8AB0,x[$0F:8AF8] A:0000 X:0048 Y:0000 P:envMxdiZc
$02/CBB3 99 FD 74 STA $74FD,y[$7E:74FD] A:0044 X:0048 Y:0000 P:envMxdizc
$02/CBB6 C8 INY A:0044 X:0048 Y:0000 P:envMxdizc
$02/CBB7 E8 INX A:0044 X:0048 Y:0001 P:envMxdizc
$02/CBB8 C0 08 00 CPY #$0008 A:0044 X:0049 Y:0001 P:envMxdizc
$02/CBBB D0 F2 BNE $F2 [$CBAF] A:0044 X:0049 Y:0001 P:eNvMxdizc
$02/CBBD 7B TDC A:00FF X:0050 Y:0008 P:envMxdiZC
$02/CBBE 99 FD 74 STA $74FD,y[$7E:7505] A:0000 X:0050 Y:0008 P:envMxdiZC
$02/CBC1 4C A0 BC JMP $BCA0 [$02:BCA0] A:0000 X:0050 Y:0008 P:envMxdiZC
; load six character spell name ------------------------------------------------
$02/CBC4 AD C8 34 LDA $34C8 [$7E:34C8] A:0023 X:0004 Y:6400 P:eNvMxdizc
$02/CBC7 85 26 STA $26 [$00:0026] A:0023 X:0004 Y:6400 P:envMxdizc
$02/CBC9 A9 06 LDA #$06 A:0023 X:0004 Y:6400 P:envMxdizc
$02/CBCB 85 28 STA $28 [$00:0028] A:0006 X:0004 Y:6400 P:envMxdizc
$02/CBCD 20 60 85 JSR $8560 [$02:8560] A:0006 X:0004 Y:6400 P:envMxdizc
$02/CBD0 7B TDC A:0034 X:0004 Y:6400 P:envMxdizc
$02/CBD1 A8 TAY A:0000 X:0004 Y:6400 P:envMxdiZc
$02/CBD2 A6 2A LDX $2A [$00:002A] A:0000 X:0004 Y:0000 P:envMxdiZc
$02/CBD4 A9 03 LDA #$03 A:0000 X:00D2 Y:0000 P:envMxdizc
$02/CBD6 99 FD 74 STA $74FD,y[$7E:74FD] A:0003 X:00D2 Y:0000 P:envMxdizc
$02/CBD9 C8 INY A:0003 X:00D2 Y:0000 P:envMxdizc
$02/CBDA BF 00 89 0F LDA $0F8900,x[$0F:89D2] A:0003 X:00D2 Y:0001 P:envMxdizc
$02/CBDE 99 FD 74 STA $74FD,y[$7E:74FE] A:003F X:00D2 Y:0001 P:envMxdizc
$02/CBE1 C8 INY A:003F X:00D2 Y:0001 P:envMxdizc
$02/CBE2 E8 INX A:003F X:00D2 Y:0002 P:envMxdizc
$02/CBE3 C0 07 00 CPY #$0007 A:003F X:00D3 Y:0002 P:envMxdizc
$02/CBE6 D0 F2 BNE $F2 [$CBDA] A:003F X:00D3 Y:0002 P:eNvMxdizc
$02/CBE8 7B TDC A:0081 X:00D8 Y:0007 P:envMxdiZC
$02/CBE9 99 FD 74 STA $74FD,y[$7E:7504] A:0000 X:00D8 Y:0007 P:envMxdiZC
$02/CBEC 4C A0 BC JMP $BCA0 [$02:BCA0] A:0000 X:00D8 Y:0007 P:envMxdiZC
chilly, all you're interested in is this:
$02/CBAF BF B0 8A 0F LDA $0F8AB0,xThis is the code loading the eight character names. The offset is in SNES LoROM SlowROM format (WindHex can show offsets as this, there's Lunar Address, probably other programs to convert if you're not familiar with it). All you need to do is change that to another offset where you've placed the eight character names and you're good to go.
JCE, this isn't going to be as extensive as what J2e did. If you want the text window changes and ridiculously long names, you're on your own. This'll give normal spells eight characters when cast; they'll still use six in the menu.
$02/CB9B C9 48 CMP #$48
$02/CB9D 90 25 BCC $25This is what's checking if the spell value is below $48. NOP it all out ($EA).
$02/CB9F 38 SEC
$02/CBA0 E9 48 SBC #$48This is subtracting $48 from the spell value. This is because they started the eight character names at index 0 to save ROM space. NOP it out as well. Conveniently, the previous code runs directly into this, so you're basically going to 2/CB9B and typing $EA seven times.
$02/CBAF BF B0 8A 0F LDA $0F8AB0,xSame as chilly's example, except you've got to have all of the spell names at the new offset.

Remember that the spells start with a blank entry. Doesn't matter for chilly's hack (because everything was already set up to handle it properly), but JCE, you're starting with a blank eight character entry, not <white>Hold.