Is the frame going to set level?
Is the rear axle installed?
What is the driveline angle compared to the crankshaft and rear pinion?

This would be a step best made once you have the chassis about completed. Most factory built cars have the carburetor flange on the intakes setting almost level, the engine and transmission will be slightly angled towards the pinion. The shorter the driveline you have, the closer you need be on getting the u-joint angle with-in the recommended 1 to 3 degrees of working angle.

For now I would tack weld them in place knowing you may need to relocate in the future. Have you thought about cab floor clearance and radiator placement?