if the convertor is for a aluminum mid plate, the converter is going to be 3/16" longer than a stock convertor. You should have roughly .100" of clerance out of the pump.If your converter is all the way in and is bound between the flywheel and front pump housing you have a few options.
Look at the pads where the convertor meets the flywheel and centers in the flywheel, they should have a 3/16 piece of steel welded on, cut them off.
Make sure the flywheel is on the engine correctly.
If its a powerglide, the input shaft may not be all the way back in the tranny, the wedding ring might be holding out a lil bit.
Lastly do some clearancing to the snout......Caution!!!
What kind of tranny is it and what has been changed on the combo?