Just curious ... any downside beside a lot of vibrations?
Noise and vibration are the two biggies for the downside. Most serious races have their engines solid mounted to the frame. The motor mounts are just there to make it more comfortable for the driver. If you aren't worried about comfort, solid mount is the way to go. I'm sure you have to go around and tighten things once in a while as well. Vibration has a way of loosening things.
You're not going to be able to weld the block to the frame, due to the block (likely) being cast. You can bolt it with solid motor mounts, which are commercially available for more common performance engines.
It would be very expensive to replace if the frame or the engine get damaged. FYI, many motorcycles, if not all, have the engine directly bolted to the frame. The main downside I can see is the vibrations.
I see this post is old but I am working on a very unique project right now.
I am putting a 1976 Buick GS 350 mod motor in a 1990 Chevy C1500 pickup. The resolution to motor mounts? 1/2 inch steel plates with direct bolt in.
Yes, it will work. It's the fastest way to bypass looking for mounts made for such a swap. Trust me I know, after searching for almost two months and looking for a solution, forums, calling custom shops all over the country, Jeggs, Summit, yeah, nobody has them, nobody makes them unless you want to pay big bucks, and when working on a less than beer budget, direct bolt-in is the only way to go. Plus it's more solid so I guess it's a win-win.