Most antenna's and recievers are made for 50 Ohms coax.
Your 75 Ohm coax will hava an swr of 1.5 at both ends causing an little loss.
If you only wish to recieve on 150-170 mHz then little gain is to be expected by swapping the cable to RG 58.
Just an little gain can be had by using LMR 400, but on higher frequencies LMR 400 will show an bit better low loss.
50 Feet isn't that long, my cables for 2/70 are Ecoflex 15 , 1/2 inch very low loss coax, 75 feeet long an tad better as LMR 600, used to feed the 11 element Flexa yagi on 2 and the 23 element Flexa yagi on 70, and the Diamond X510N.
For the 5 element 6 meter beam it's Aircom Plus, like LMR 400, same cable is used for the Imax 2000, the Diamond scanner antenna used to feed the Ft100 for local repeaters, and the FD-4 OCF wiith added section for 160 meters.
Your antenna system is as good as the worst component.
73,