Hey, I'm kinda exhausted right now, so I'll post the quick answer now, and the long, typical engineer answer later.
First, 2 inch, 1/4 wall box tube is your friend.
Tha's a bad ass land and sea dyno set up right there. I think it lists for about $1500 minus the water brake, battery and cooling tower.
I built almost the same thing for 50 bucks for testing motorcycle engines. It has a 2 inch box frame base, casters and an AL pan on the bottom where we mounted the gas tank and battery, just like in the pic.
The motorcycle engine itself was mounted in a cradle and then bolted to the base frame. In this case its a 2002 F4I Honda.
Well I'll post the long version later, and I'll see if I can dig up the pics and Acad drawings of the stand.
In this pic the cradle is just resting on some crappy table. I can't find the pic of the bottom stand yet, hopefully I can find it soon.
Overall, making it fit and compatible with your space and water brake is what is important. Some 2 inch box tube should make an awesome base and you can just build up from there.
The other system I worked with is what I had inherited. Think 2 12 inch steel I-beams with a 1/2 inch steal plate bolted on top. God awful heavy but effective. Some 2 inch steel legs were welded up to connect the engine to the base. Thats a turbocharged F3 on it.
