Check out my thread on 400+hp 273 in my signature below I haven't built it but there's some good info. I was thinking of going 3.58" stroke but I don't like the bore to stroke ratio I know there's a hyper piston for an Holden engine that will fit a 273 with a 3.58" crank but it got Chevy pin size, could use Chevy 6.125" rods by turning the crank down or bushing dodge rods for Chevy pins. But I think custom piston are the only way to go. I wouldn't worry about stroking a equally built 301 make the same power as a 273 only 600rpms lower and will make a little more torque like 20-30lb-ft but step up the rear gear one size over what the 301 would need and all would be equalled out.
I don't think you really need to run bigger valves look at 1wild&crazyguys sticky on porting 318 heads with stock 1.78-1.50 valves they flow enough for 450hp, people have run 2.02 but I think that's to much but I'm temped to be able to run eddy heads for weight savings, but EQ would probably the best bet if you don't want to port the stockers and just bolt on aftermarket heads, RHS are too heavy.
What's the power level you looking for?
Is this race only?
