You can get yourself some oem replacement rocker studs for that build at most auto parts stores (might be special order) or get some ARP brand rocker studs. Lengths of the studs themselves can vary, and depending on which rockers you ultimately choose may dictate which length studs you need.

Rocker arm kits usualy come with polylocks to lock down the lash adjustment. Guide-plates are typically sold seperately.

Can't really estimate HP, but certainly 400HP. Lots more HP is possible with that group of components you have, but head porting (particularly the exhaust ports), bigger carb and other key ingredients would be necessary (all of which might push you in the high 400's). Pump gas premium will be mandatory with 11.4:1 c/r, and blending some race gas will likely be necessary.

I know little about the transmissions, but beware that Ford offered the C6 with two differnet crank pilot diameters in the converters, which must corresponed with one of two different crankshaft pilot hole diameters. Be sure you have the correct converter pilot dia to be received by your crankshaft.

Your combo in that relatively light car with a 3.50 rear axle ratio will be a lot of fun on the street.

Paul