I just looked at a Valiant shell. Contemplating possible a BB. How do I find an point(s) where I can begin to mount the motor plate and keep the motor, trans and rear in alignment? I use to use the factory motor mounts as a guide when mounting a plate in the past but nothing than I know of now.

Install the engine,trans,headers and use a jack/atv jack under the engine to hold it up or an engine hoist then mount the plate on the engine and mark things.

Use the transmission mount as the locating feature and work from there. Slide a long tube thru the mains and the transmission case. Line it up so tube points to the pinion then you lock it down.

Before you lock everything in place it is a good idea to bolt on a set of heads and install the headers. Then install the suspension and make sure the center link and tie rods clear the headers. Putting a big block in an A body car requires a lot of little adjustments, especially if it is a performance application with larger tube headers. You'll really want to spend some time making sure you have room for everything before you weld the tabs to the frame rails.

That's some good info. There's no rear in it. Usually I use the motor and trans to locate the rear. So, do I use the motor/trans pipe system and locate the rear after I've bolted the motor and trans in first?

I wouldn't do that Use the trans tunnel center for alignment on the trans and make sure and do as Andy suggested, use the headers to locate the engine in the engine bay

Well you can but locating the rear is more complicated than that. Typically you need the rear end to be centered within the frame and the body so you have proper tire clearance on both sides of the car. If I'm building a car I'd worry about that more than perfect alignment with the engine and trans since you have u-joints in the driveshaft to pick up the play.

Of course, all this depends on the car. A tube chassis car is a little easier to build than a car with a stock floor. On a 50 year old car with a stock floor you often have no idea where to find a datum. Nothing is straight on those cars after 50 years of abuse so it gets harder to figure out what exactly you are squaring things up to.

Thanks to all!

Find the center line of the car first thing.. the engine and trans is normally about 1" to he pass side
of the car for driver clearance.. if you use the center line method it will help in building the whole car..
everything on a car is based on the center line