I'd say stay away from the turnbuckle idea. I had an aluminum body turnbuckle with the eyelets on the ends on my race 289. I think the motor made somewhere in the 400-450HP range. After every pass I had to put another couple of turns in the turnbuckle as the eyelets kept stretching open. What I would do if I were you would be to take a couple pieces of angle iron that are as long as the frame is wide. These pieces would be back to back running across the frame. Between them would be a flat strap of metal that runs up to a head accessory bolt hole. You could weld the angles to the frame or drill a hole thru the top and bottom of the frame and use a bolt that runs all the way thru. I'd put the heads of the bolts on the bottom of the frame, that way you can see if the nuts are working loose. It's not all flashy or anything, but it's effective and cheap.