What do you think a fair price is for my 1967 318 engine

I think Memike is on point, I would ask $600 in Houston and negotiate from there.

In south Texas allot of people ask just the sane side of outrageous for parts(i.e. sometimes only $1500 for a running 50 chevy pu or $4000 for a rusted, dirt-sunk Duster . I believe they do that as a starting point for low-ballers.

You have the dress up kit, it runs well?-good things to have going for you.

People are going to offer less than half of whatever price you start at $600 or higher, and let them work their way up to what you think is reasonable (above $400??)

I would have a handy list of the going price is for new rockers, push-rods, lifters, gasket set, bolt set, heads & machine work, block & machine work, pistons, rings, connecting rods & bolts, oil pan, oil & water pumps, intake, carb's all run for together as a basis to go off of/argue with low ballers that try to break down your price as too expensive.
These two techniques (low balling and mark up) seem to be too common now.

If, someone seems to come and offer a straight, and fair price initially, none of the fore-mentioned negotiating will be necessary. I would not put firm (turns people off) or OBO or best offer {encourages lowballers} in the add.

The moment anyone became manipulative, in heavy negotiating, I would tell them to have a nice day versus striking a deal and them coming back for a refund for lord-knows-whatever-reason. One particular dealer here has made it difficult for honest buyers-possibly avoidable if you talk to someone and get the vibe from them.

just my ten cents, good luck