I'd like to offer a warning on your engine and trans mounts.
The route you're taking is not sound in design. The front horseshoe mounts like you're using require a bellhousing type of trans mount. That's the way GM did them for a reason. Your big truck with that type engine mount has two big bellhousing mounts does it not ?
The span from the front engine mount to the trans mount on the TH350 is considered too long (by smarter engineers than I will ever be). The entire length of the powertrain combo puts a lot of stress on the aluminum transmission housing. Think of a long board and how it sags in the middle.
If you'll notice on factory GM vehicles with front engine mounts, there was found a bellhousing support of some kind. The shoe box 55-57 guys deal with this a lot when they swap auto trannys into their cars. An added concern in your configuration is that the front engine mounts being somewhat narrow distance apart allow the entire engine/trans combo to roll on it's centerline under torque. The wider spaced later type side mounts stop this from happening, or the bellhousing mounts stop it, one or the other, your choice.
As an example, here are links for auto trans conversion for a front mount GM cars. Notice they include bellhousing mounts that work with later auto trans swaps.
http://www.classicchevy.com/chevy-tu...1955-1957.html
http://www.danchuk.com/ItemForm.aspx?Item=870