Your strongest frame connector will be a rectangular box iron with 4 sides, with the widest beam side being vertical with the other frame members in the vehicle, if cutting the floors doesn't bother, then cut the floors so that the connector protrudes thru the floor, once welded from both sides, they will appear as the laser cut floor matching units sold, only stronger, you could easily fit 2X3 or 2X4 inch box iron connectors doing it this way