So...you are getting a 3070 that has no CPU installed, which is a used system I am guessing? And according to the specs on this page the 8400 is not compatible with the Micro. I suspect the reason may be the higher power draw and heat. So will it work real world, possibly not but the only way to know would be to install one and try it. This forum thread has information however on installing the i9 9900T but it is a pricey CPU.
I have just read online that the i5-8400 CPU is better than the i5-8400T so I was wondering if I can use it it the Optiplex 3070 micro that I am expecting without a CPU installed? I am rather new to this kind of thing so forgive me if I appear ignorant.
Optiplex 3070 micro