I built mine out of both angle iron and 2x4's. Plates for wheels welded to angle iron. Angle drilled and 2x4 frame mounted with lag bolts.
I've had my engine and tranny bolted together on the top.
It's 25" wide by 41" long and with wheels 34" tall. to the outside of the 2x4s
I think it would need to be a little wider to slide the tranny bell housing in on the lower level if you wanted to swing it straight in there. I just slid mine in sideways.
I have a vice mounted on it as well.
Here's a pic of engine and tranny on the cart, it sits on the HE's nicely:
