Some home brew last night I have this type U310 J-FET also for 50MHz
a very stable and low noise design (N6CA)
I could not tune the input circuit tried all kind of L/C combinations
The amp seems to work okay
But saw no S/N improvement on the IC706 144MHz receiving.
Already there is an "antenna" noise increment upon connecting the antenna.
So improving S/N is not possible anyway this way.
Also found that my LED reading lamp IKEA has a nasty electro smog while having the preamp close to it on the desk.
No HF smog observed until now.
Also 144 had a very bad drifting rattling noise moving over the whole 144-146 MHz from the south.
DeJa vue from my 2m magic days long time ago when I was an active 2M DX'r
we alway had to fight noise sources even here in the country.