Did you by any chance buy your mount from Speedway Motors? That's where I got mine. I'll tell you what I did to locate mine. Because I wanted to use the original SM420 transmission with the 350 V8, I located mine with the transmission in place. I mocked it up, with the engine hanging and no clutch, and slid it back until it mated with the bellhousing. Put in a couple of bolts finger tight to hold it in place. Then with the motor mounts on the engine, I had to estimate right about where the tubular mount would bolt up to the frame rails. Then I pulled the engine back out and measured that. I cut the tube as close to my measurements as I could, erring about an eighth of an inch on the long side, and then hand-ground it little by little until I had a good interference fit.
Then I lowered the engine back in, mocking it up again, but this time with the tubular mount bolted onto the motor mounts, and located my holes I needed to drill for the pieces that mated the tubular mount to the framerails; 3 bolt holes for each side.
One caution here. Although it's tempting to weld the mount in place on the framerails, DON'T!!!!!!!!!!
I am so glad I didn't. There is VERY little wiggle room in that narrow engine bay. With the clutch in place, the only way to go is straight back. If that tube was welded to the frame, it would have restricted the engine moving back. You have to leave everything loose so that you can swing the mount out of the way as needed when sliding the engine back into place. Then with the engine still suspended by the hoist, torque all the bolts into place.
It's a [censored], but it works well when you're done.
