It's not too bad if you don't have A/C, otherwise you'll have to edge the compressor out of the way after loosening the drive belt tension, special 'sawn off' allen key required. And whilst it is a 'simple bolt on' job, you'll still swear and curse. And yes, you need a new gasket, around £15, manifold is maybe £300, there's been a guy on eBay selling a few recently, make sure you get the final design with a groove in the flange between cylinders 4 & 5. There's a guide on KayJeys's site IIRC
http://www.fiatcoupe.net