Well greys are good for 450 no more so that's all your ever going to see.

A t38 will make it easier, just keel the boost down for reliability

I'd go with a good head gasket and go from there a t34 at 380 on a std engine with a good head gasket will be bomb proof though, proven for years and like I say fab conversion so up to you, either way will be fine on STD engine