Agentadds,
I have managed to install both of the saddle tanks in my very early turbine conversion BVM T-33, and have found that some modification is required in order to accomodate the tanks in my early airframe. Adequate clearance to install the tanks was non-existant. My tanks were early models that had fairly thick flanges, and required some heard scratching to make them fit.
When I bought my T-33, it had been started, but unfinshed. It was originally a ducted fan airframe, but had the turbine inlets installed, and contained all of the fuselage frames and formers from the original build.
Here's what I had to do to make the tanks fit without jamming them in and hoping for the best.

1. I found that the original frame F-2, located on the bottom of the fuselage, interfered with the saddle tanks. Most of F-2 had to be removed. Removing most of F-2 will not result in structural problems or weakness.

2. The main fuselage frame F-3, which is made from 1/4" plywood, needs to have reliefs ground in it to clear the aft end of the saddle tanks. The drawing top view shows the tanks extending through this frame, so I didn't feel too bad grinding some of it away. See the attached photos. F-3 is massive, and is overbuilt. Tank reliefs will not compromise the strength.

3. The left tank was pretty thick in the joggled area that forms the tank half assembly joint. The photos show how I thinned (tapered) the flange to afford more clearance in the fuselage skin and inlet tract areas. There is still more than enough material remaining to make a leak free joint. The right hand tank flanges were thinner, but still required some judicious material removal.

After accomplishing these changes, the saddle tanks fit the fuselage without deforming the skin.

I used a pnuematic 90 degree die grinder with a sanding drum to remove the material, as a Dremel tool was too large to fit the work area.

The AFS version of these kits (F-80 and T-33) are totally redesigned and should not exhibit the same condition.