[BR3W CITY]

Position is slightly subjective, there is no "right" answer. In general, if you look at the stock position; anything within about 1" forward or aft will be fine. Going further forward than that will make radiator/fan/front acc clearance tight, and put the weight further past the axle centerline. Were you to go 4" forward, there would be a noticeable change in "push" with the handling.

1" back is usually the most anyone can go without hitting the coil packs into the firewall/trans tunnel. Again, some folks really want to put the weight further back, but make no other changes to the chassis to benefit from it...rendering it fruitless.

Your exhaust manifolds won't fit, 90%+ of swaps don't have room between the rails to fit the "bumps" on the truck manifolds. TBSS mani's, new camaro, tubular fbody etc have all been made to fit.

Your 4l60e won't have the same splines as the 400. Personally I feel that a custom driveshaft is always a good idea (yolk type and spines will be correct, length will be correct and you won't have to sacrifice driveline angle).

As for the shift level, are you referring to a column shift or floor shift auto?