It all depends on what brand headers you purchased. If you got Flowtech headers, then yes, they require hammer fitting in most cases...cheap quality. With any other brand you should be fine. If they're not Flowtech headers then your mechanic is nothing more than an imbacile and shouldn't be allowed to work on any motorized product. Don't revert to buying shorties if you already have the long tubes and the matching h-pipe.
Are the headers new or used? If you purchased them new, I would check the part number and verify that they are correct for your application before hammering on them or just trading down to shorties. I installed Flowtech full length headers on my car and the pass. side fit well, but the driver's side had to be beat to hell. I'm saving for better quality headers so I don't have to f*ck around and I can make more power.
New Toy- Stone stock '88 T-bird Sport in need of some TLC
1988 Cougar XR-7- HO/T-5 conversion, cracked block prevented from reaching full potential.
RIP '88 T-Bird....firstname.lastname@example.org...best sleeper in So. Jersey, now it's taking a permanent dirt nap.