On a budget, the Edelbrock 5090 E-street 75cc heads are a good choice. The 440 source stealth head is decent if you want more of an original appearance. Both can reuse the stock rocker gear if using a mild flat tappet hydraulic cam. If trying to save money, use a cam that will work with the valve spring specs that come with the heads.
With 75cc heads, use the 27cc dished pistons for about 10:1 compression.
If using 84cc heads, the 17cc dish piston is about 10:1 compression also.
Something like the Comp Cams XE275HL would be pretty tame in a 500" engine. I haven't verified it, but I think the cam would work with the Edelbrock valve springs. You could use a larger cam for more power, but most likely will need to replace the valve springs.