Originally Posted by Byubuk
...Can i get by with building up where the tub will sit and connecting new tub to old drain without busting concrete?...
You will have problems.
There will be an increased chance of odor in the tub drain, problems cabling the drain in the event of a blockage, and the tub itself will not be as stable as if you install it on the slab. With cast iron piping under the slab, having access for maintenance is critical. The biggest problems with cast iron drains serving tubs are:
1. Getting access to the drain through a cast iron or lead p-trap.
2. The p-trap itself.
Both of these problems get a chance for a reset with the proper installation of the new tub.
It will be much easier to remove a bit of concrete and relocate the drain than building a platform solid enough for the tub. You also avoid years of having an extra step up to the tub. I see DIY installs all the time like you are describing. They NEVER last and the customer is NEVER pleased with the result.
This is truly a case where it is easier to do it the right way.