Perry,
Great to read your post on your venture into casting.
I'd say the .358 sizing die you have should be OK in your 35-303, I'd certainly try it out first before I bought more.

You may find that the wheelweights on their own, may not allow you to get to 2100-2200 fps without a touch of leading. With the pure lead you have, if you don't have a need for it, say for a muzzleloader, I'd try and swap it for more wheelweights or some linotype, to harden up the wheelweights slightly.
You may also try dropping the wheelweight bullets out of the mould into a bucket of water, give them a few days, and they should turn out harder than the non water quenched ones.
Wheelweights quench very well, no need for the extra linotype in it then.
Put a sponge on the bottom of the bucket so the hot bullets don't hit the bottom of the bucket, but land on a sponge.

One very big warning though, DO NOT splash any water into molten lead, or moisture from anywhere, it turns to steam instantly and will literally explode, covering you in molten lead.

I would like to suggest to you to keep asking questions here if you need to, there are a number of very experienced bullet casters on Nitro Express, and we are all here to bounce questions and ideas off one another.

An old boss of mine used to tell me, "There are no such things as stupid questions, just stupid mistakes".
If any of us can help, we will.

And lastly, post photos and tell us how they go for you!!

Regards,
David.