PcLinuxOs KDE 64 bit - bootloader on the same partition

[Apanta]

Hello.
I downloaded and mastered PCLinuxOs 2017 - 64 bit kde.
I have this situation on my pc:
sda - data disk - ntfs
sdb - data disck - ntfs
sda in which there are:
- sda1 - swap
- sda2 - extended partition containing inside:
- sda5 - Ubuntu 16.04 - ext 4
- sda6 - Data partition in ntfs
- sda7 - ext 4 partition where I desire install PcLinuxOs
- sda3 - Win 10 partition - ntfs

Currently I have Ubuntu 16.04 installed in sda5 and Win 10 installed on sda3.
The boot is managed by the Ubuntu grub, installed in sda.
I launched the installation of pclos following the directions on video, allocating it on sda7, but it does not give me the opportunity to decide where to install the bootloader (which I want to put in sda7, and then manage everything with the Ubuntu grub).
In fact, proceeding with the installation, a window appears where I can choose the position of the bootloader but, strangely enough, it leaves me only three choices:
sda where Ubuntu grub2 is installed.
sdb - data disk
sdc - additional data disc.
It does not let me choose sda7 which is the partition where I installed pclos.
I do not understand why I can not install the bootloader in the same partition (sda7) where I want to put PcLinuxOs.
I have therefore aborted the installation and here I am to ask you a little help.
Thanks in advance

ps. sorry for my poor english

[oldfred]

If it uses grub to boot, install it to the data disk. It only writes to the MBR and a few sectors after the MBR (if MBR(msdos) and not gpt).

Or you can install to drive, and then load Ubuntu and reinstall its grub to MBR. Or use Boot-Repair to reinstall grub.
https://help.ubuntu.com/community/Grub2/Installing#Fixing_a_Broken_System

Boot Repair -Also handles LVM, GPT, separate /boot and UEFI dual boot, only use ppa download into Ubuntu live installer.
https://help.ubuntu.com/community/Boot-Repair

[Apanta]

Thanks for replay.
My fear is to compromise access to the data disk (eg sdb) if I install the pclos bootloader on sdb

[Apanta]

Ok thanks to all.
solved thanks to a friend (dxgiusti) as per link.
https://www.dropbox.com/s/5y34r6pyeolcmfc/dualboot.mkv?dl=0