Need to determine ideal holster for X-frame 8 3/8 with Allchin mount and Vortex Venom red dot. Will I need a holster capable of a true scope mount? Would prefer modern material (nylon) and chest cross-draw position.

While I can not give you a specific brand or model number (don't own an X-Frame) I would make sure which ever one you decide on has adequate weight dispersion qualities and maybe even padding on the shoulder strap as those guns are real heavy. I would think that the wider the shoulder strap the better it would be at doing this. I can only assume you are using this as a hunting rig and will wear it exposed and in full view so concealment should not be an issue - hence the size and width should not be an issue either.

My .460 came with a nylon/camo chest holster that works with a scope mounted, but it was essentially a bucket. Didn't feel comfortable or ride well, although it did protect the pistol and scope with a full size flap cover. Talk to the crew at Diamond D Holsters in AK. I'm a bit biased because I've used their products for years. I have them for short N frames, my .460 X frame, a Sig P220 in 10mm and now my Glock 40 long slide in 10mm with a red dot mounted.

The holster pictured below was made by them about 5-6 years ago and will accommodate my .460 with or without scope, so it will work for your needs. It's leather, so it'll get wet and take a bit longer to dry out than nylon, but it's durable, comfortable and works for me.