Six paks rock
I've built, owned, race and abuse more than one motor combination using them
As buying a new crank and rods I would buy a 4.25 stroke stroker kit from your favorite parts vendor, like 440 Source
There are othhers out there also
I buy my stroker cranks from Ohio Crankshaft Company with the BB Chevy 2,200 size rod journals and good brand of 7.100 long forged steel H beam rods from a good souce like Molnar, Scat, Ohoi Crank CO or others, your choice. Piston will need to be made to match your combination to make sure ou can safely run todays pump swill
If your going to use iron heads mske sure to keep the compression ratio at or below 9.5 to 1
I built a similar combination, pump gas, 906 heads, six pak intake and stock OEM type vacume 1970 carbs, it was a 400 block ,505 C.I., with a Eddy low deck sixpak intake, it had a custom ground Comp Cams solid roller cam and roller lifters in it.I had several different sets of six pak carbs at that time, I sent one stock set off to C&J Engr. to have them custom build them for my motor, they made 605 HP and 640 Ft.Lbs torque on a DTS engine dyno in Ontario, CA on CA pump swill back in 2002 or so
Lots of choices out there, have fun
. My next one & what I would suggest is a 4.5" stroke crank/rods/pistons from 440 source (as you said it'd be hidden). 906 heads which are what you had originally are readily available used & with a stroker they would hold you back but you could have em ported to the max. the only issue (for you) is the 4.5 stroker requires external oiling which'd be a pickup in the bottom side of the pan and a braided line to the oil pump. Not sure what the strok(er) limit for not needing that is but 'source would know. Not sure what balancer you had originally but someone here does & with a stroker you'd just have it in the parts pile given to your crank guy when you have it all balanced