Giddy Jason, sit the engine and gearbox into place carefully checking all the clearances, the steering arms, firewall, floor, starter motor, radiator, tailshaft alignment and length (It's an advantage to fit the tailshaft up temporarily at this stage) etc. then make up the mounts and sit them into place, I'm pretty sure the standard mounts were right at the front of the crossmember and the new ones will be at the rear, let a bit of weight on the mounts so they sit firmly, RECHECK EVERYTHING, then tack mounts into place, remove engine or front end add a couple of triangles of metal on the sides for strength and then fully weld, easy as
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Pedro
p.s. yes I know I didn't use triangles on mine but i made them long enough so i could weld the rear to the side rail.