Small bits in very large routers using an edge guide ? = you are much braver than me. I would never use a 1/4" bit in a 3.5 hp router and I can only think of a few times when a 3.5 hp router would have been safe when using it hand held. This size router belongs in a router table where you can use a real and substantial fence with it. The largest router that I'll use hand held is a 2 1/4 hp. I have 3 DeWalt DW618 router kits of this size - 2 three base kits and 1 two base kit.
If you are determined to proceed with this, look for an off brand router fence that will fit your router. Most router fences attach using the horizontal holes above the router base. The fence attaches via rods that slide into these holes. The hole diameter and spacing of the router needs to be known to make this decision. Router base plates are relatively easy to make yourself using good plywood wood or Lexan, a shatter proof plastic, or you can buy a base plate with many holes to make it adaptable to most any router of that size and screw hole pattern. Collet bushings will adapt your 1/2" collet to accept 1/4" router bits.
Charley