Can you fit 3.5 heads to a 3.9 block as i have a good 3.9 block and a 3.5 with u/s crank

Yes, straight swap,ignore the extra 4 small bolts if the block is not drilled for these, not required.

The newer stretchy bolts feel really horrible when you tighten the heads!

Regards Mark

You might find the 4.6 block is the same size but lighter.
I once had John Eales looking at the best way to increase power to weight for my Westfield and this was the first change we made.