^^ Actually the 4.3 V6 has the same mounting location as the small block chevy. The only diffence is that the 4.3 is missing the two front cylinders! Yes you can use the V8 mounts on the 4.3.
diferentone- that is the same croissmember I used to mount my SBC in my mazda I also got it from Summit. You need to get say 1969 camaro engine mounts to use that crossmember. Heres a pic of my mount, it is set way far back, yours would most likely be set more forward.If I did a 4.3 I would use the exact same mount.
PM me if you have more questions.