This is a cool project! I wouldn't stress too much about getting a chassis fixture as long as your work surface is flat and level. We have sold thousands of chassis kits that were built in garages around the world. Just keep measuring three times and weld once.
Regarding the engine mounting, if you have a front plate you definitely need to run a mid plate. It's going to reduce binding, increase safety,
and increase chassis rigidity. Also, you should run a torque limiter. This will keep you from cracking the tranny case, or the billet front plate.
You weld the stub coming off the rod-end to the frame rail then the other end bolts to the engine mount boss.
