273 time to 'fish or cut bait'...
I pull the intake and the heads and take the heads to the machinist to make sure they aren't cracked (He will also clean them and put in 'hardened seats' and promised to 'port' them for me). Then he tells me that the valves are in bad shape (see photo) and they should be replaced.
With just that little bit of info...I'd seriously be asking myself, "What's the end goal?" I have not doubt your machinist can work the heads over for you but, labor and parts is not cheap! I'd bet that the valve guides are shot too. Now you are looking at cleaning the heads, checking for cracks, new valves, new guides, install seats, new springs, a valve job, and probably shaving them a little to clean up the surface. In short...you are going to have a LOT of money just the heads (I'd guess at a bare minimum $800+)...and that's not counting what he may charge for porting. Cheap valves for old 273 heads...use 305 Chebby valves (1.84/1.50 and 11/32 stem)
Pistons...Hughes does show they still have the Egge 4-barrel cast pistons...
Hughes Engines
...but, rings are getting harder and harder to find in 3.625" and 3.655". And intake wise you are VERY limited...without modifying the bolt holes on the manifold you use. EVEN if you find a factory intake...you are limited as to what carb will bolt to it. Your 273 rocker arms should be adjustable, should be reusable, and will take a beating.
Don't get me wrong...I'm a 273 fan myself BUT, if you are just wanting it to play with...I'd go 318. For starters, you get 45 extra cubes...16.5% more = more power all over. The 318 is only 22 cubes off of a 340! Blocks are CHEAP and even close at hand..
318 motor
Decent pistons are much easier to get these days! Wiseco makes a forged set for around $500, comes with rings and pins. ICON has a set for $650....or KB Hypers are $300-400. Easier to find intakes. Rods...just get a set of Scat I-beams with the basic bolts...no need for ARP2000's since you'll already be going from 3/8 factory bolts to 7/16" in the Scats. Those can be found for $350 or less...just a few dollars more than refurbing your old rods AAAAND they'll save you some weight. Heads...
IMMENGINES.COM
the LA version is what you'll need and those should handle ANYTHING the 318 will need! They might look like they cost a lot...but, have your machinist price out the work, and all new parts, on the old 273 heads, it'll be a lot closer than you think!
The only really noticeable difference on the outside of the 273 and 318 is going to be the intake bolt angles. Reuse the timing chain cover, valve covers, etc., from the 273.
Lastly...the crank. The 273 will have a forged crank and the 318 is cast. If you are going with a 4-speed, it's an even swap and I'd use the 273 crank in the 318 build. If you even remotely think you might switch to an automatic down the road...have the machine shop enlarge the register on the back of the crankshaft to match that of the 318. The early 273's used a smaller register and it makes it harder to find a torque converter.