the above guide is for uefi + gpt drive, you may check here for mbr + legacy Qubes OS Installation - BTRFS w/ Detached encrypted boot and header
you can notice we have different kernel parameter, this is the parameter in legacy, full command on link above.
cryptdevice=aa7332a5-e4ac-442c-9328-cdbd4a0b42e8:luks-aa7332a5-e4ac-442c-9328-cdbd4a0b42e8
also im not familiar with how heads boot, so try hard.