I have heard of people using a Cometic DOHC head gasket and just trimming off the end-piece that covers the timing cover and replacing it with one from their stock head gasket or making one from silver McKanica. aside from the timing cover portion of the gasket, the DOHC and SOHC head gaskets are identicle, and all that piece is, is an oil seal.
I second the reccomendation to NOT use TopLine timing components. A good friend of mine did a rebuild of his DOHC Ka and the tensioner went out within a very short period of time (within break in period) as well as his topline oil pump. LOTS of people over on
http://www.ka-t.org have had numerous issues with them as well. I would reccomend going to the Courtesy Nissan website as they are a sponsor and ordering the OEM Nissan timing parts.
You can also order an OEM oil pump for an 81-83 280z (L28ET engine) It runs a little higher volume and is a direct bolt on for the SOHC. I plan on running one because the oil feed line and oil cooler I'll be running will put more of a strain on the oiling system than I feel comfortable letting the OEM pump handle. I don't know if you plan on running an oil cooler but I would highly reccomend it as they are cheap insurance for an expensive engine build.
I should be doing a SOHC build up within the next year or so for turbo and plan on doing everything I just listed as well as some extra goodies. We'll see how finances pan out though.
I'm not sure how balls-out you're planning on going with the engine but you can ask how much they want to balance your rotating assembly. basically they'll take your flywheel and clutch and assemble your entire short block and balance it out. It isnt neccesary for normal street cars but if the price is cheap and it's in there already, it's an added bit of insurance as well. (I think my friend paid 80$ or so to have his rotating assembly balanced while his engine was in the shop)
If you're not having the shop assemble your head as well, you might ask them how much they want to inspect your head to make sure it's not warped and mill the surface so you get a good seal w/ the head gasket. Most likely they won't charge much to check your deck tolerances, it's especially helpful if you know what the need to be. go to
http://carfiche.com/ and pick up a FSM if you don't already have one. If they need specs on what the tolerances should be in the bottom end or head you will already have them at your disposal.
I have to get going to work but that's all I can think of for now. I'll come add more if I remember anything later.