Never used a 2 bolt block and now that I need a new block, I seem to be able to find lots of these 2 bolt block and few 4 bolt blocks. Or should I use my 4 bolt main caps and machine the block for the extra bolts?

Believe it or not two bolt blocks are stronger that four bolt ones if you drill them for splayed caps(angled outer bolts not straight like stock caps)
The outer bolts go into a thin part of the web and actually weaken it. Splayed caps put the bolts into more meat and increases the integrity of the block .

I`m sure others will bark in here after my comment, but I have never seen an engine failure due to the integrity (or lack of) in the main cap area. At least not with naturally aspirated big blocks. 450 hp is not that much more than a base BB Chevy. 2 bolt blocks can handle much more. If in doubt, do as Mopower suggested. He is 100% correct in that there is more meat in the main web of a 2 bolt block. Splaying the outside bolts can greatly increase strength in this area. Go for it!

Read Dennis Moore's BBC Marine Performance book. He makes mention of the 2-bolt blocks and the use of cast crankshafts. It is the rpm or lack of, that makes both of these components okay in marine powerplants of that horsepower range.

Chevy ran 2 bolts in their BBC production engines for years. I know of 66 Vettes with 427's putting out 500HP turning 6500 with no probs on 2 bolts. As other have said 450HP is no that much, I don't think I would want to throw a 400hp NOS shot at a 2 bolt but thats another story......

I have ran up to 800 hp in a jet boat & never had a problem with a 2 bolt block

Go for it!!!

I know of several guys who ran 2-bolt main blocks in 454's for hi-perf marine applications. No need to splay the main bolts either----just use ARP studs and have the main saddles align honed as Budah said. We did this back in the 1980's with good success! I know of a guy who used a 2-bolt block with a blower making about 700hp or so in a marine application
with no problems!

So, Don't sweat it!-----Have fun!