SBC's are 3/4" or so longer so if you sat the engine using HQ 350 mounts the bellhousing mounting face will be the same as the 253/308. From memory HK-HG mounts sit it back 3/4" at the bellhousing mounting face, but I think the engine sits lower with HK-HG mounts. What mounts are in the truck?

Regardless of all that though, the trouble you may face with a SBC is a Holden V8 has #1 cylinder on the driver's side so the RH head sits further forward than the LHS. A SBC has #1 on the passenger side so its driver's side head sits back. This might make it worse. I guess you'll have to do some measuring!