I think first and foremost, we should start with the observation that, as far as I can see, u-boot doesn’t support elf64. However on the assumption you’ve got an out-of-tree patch to add elf64 I can answer the rest.
The choice of address will depend upon what is in the program header of your elf binary. bootelf will copy the image to wherever the program header tells it to… so your choose of address must not overlap with wherever you elf image will be loaded). For a 90MB executable the normal load addresses such as $fdt_addr_r or $ramdisk_addr_r) are likely to overlap with the executable and therefore cannot be used.
So a rule of thumb would be to look at the highest address in the program header (e.g. the highest paddr+size for a loadable segment) and use something slightly higher than that as your temporary address.