Rebuild or replace?

You already have the 273. I know people are suggesting a 318 or a 360 but that's gonna cost $200-300 and then you'll have an engine you know absolutely nothing about. You know your 273 burns oil and such? Pull it yourself and disassemble yourself. Have it tanked and maybe a freshen of the bores. Have the crank polished and use all your original parts. Hot tank and polish shouldn't be more than $200-250 and then new rings will set you back another $50.
Clean everything else yourself. Maybe throw in a new chain and for less than $500 you have a completely brand new 273 which you know your convertible can handle.
You'll be surprised how much you'll probable enjoy it. Good luck.