You are trying to boot off the old HDD. Your BIOS may not be set for legacy MBR.
Beside that, the best outcome would be to have the system on the SSD. Since you can boot to the USB Windows install. I would install that on the SSD and see what happens. I never like using old systems with potential registry issues, but I have used old HDDs in new systems when I knew there was little change for registry errors to get going quickly. Then it was more like will this really work?
Once you have a working system you can access the old HDD. Personally, I use my old HDD to hold my user files. This way, if I have a system issue, I can reimage my system without affecting my data files. Also, the SSD can be 256GB - for some 128GB is totally big enough.
I know, you don't want to reinstall all your programs, but with the HDD you can move program specific config files and at least you won't have to reconfigure all the programs - that is if you can find them.