Seat Covering Help...

OK - so I bought enough to completely redo the interior of my well-worn 65' Barracuda. New carpet, headliner, seat upholstery etc... I redid the rear seat with no problem - almost even look s professional. Now I am on to the front seats. The seat foam was worn out so I bought new seat foams for the front seats. Got the new seat foams and just got the inner ring to the seat upholstery installed (the part of the seat cover that has the tree metal rods and makes the "indent" in the center of the seat. Now I am finding that the seat foam appears to be too big - though it pretty much lines up with the old one, just more material.... I am afraid of pulling the vinyl too tight across the seat foams (which would require another person pushing with all their weight and me stretching the material over it. Anyone else done this and had similar experience problems??? Do I shave some of the foam off??? (Though that seems to be a bad choice) The parts came from Year One - and they seem to be the "right" parts - (IE the seat foams line up perfectly with the rails, the design, etc... but they are just too stiff esp. along the outer corners..) Help...